Some duelling tips that may come in handy and that are really useful and important:-

Know Your Opponent- As I said, duelling is an art and people, even your opponent, have their own flair at duelling, certain duelling methods are more suited to them and they are likely to use these more if they are smart, and their duelling aptitude is related to their personality. Therefore, different people are likely to have different duelling techniques up their sleeve and are also likely to resort to different types of duelling methods according to their personality. So, observe them and try to learn about them and their personality, if possible even when you are not duelling, to be able to guess what kind of duelling methods they are more likely to resort to, whether they are going to be more offensive or defensive, how confident and/or proficient they are and how that is likely to affect their attitude and abilities during duelling, what are their triggers and how they are likely to react to your means of attack and your defense mechanisms, and how their duelling flair or aptitude will likely be. This information helps you to plan beforehand or prepare, the best methods of offense to beat your opponent and the best defense mechanisms to protect against their methods. Planning ahead is always a great tip for duelling for anyone, which is our next point.

Plan Ahead- Do not wait for your opponent to strike before you plan your action. That is only going to delay your defense and make you vulnerable to attack from your opponent and defeat and/or also delay your offense methods and give your opponent time to build their defense, escape, or to think of the exact counter, or to prepare their next attack. You do not want to give that time to your opponent or you will soon have a huge backlog of planning and thinking about offense and defense methods on your hands and be caught off-guard and defeated by your opponent. Duelling is all about thinking ahead and thinking fast.

Practise Quick Thinking- Practise thinking really fast and really smoothly so that you can properly time your defense mechanisms. Also, think hard. Do not compromise your thinking just because you have to think fast. You have to think efficiently to be good at duelling and have victory on your hands while at the same time thinking fast. Your pace of thinking will need to be more than or atleast similar to that of your opponent in order to win a smooth victory. The best way to be good at this is to practise. Practise thinking quickly while at the same time thinking hard. Practise by duelling and practising duelling or by other means that sharpen your brain's thinking capabilities and your brain's agility such as certain timed word games and timed strategy games.

Practise Agility and Swift Movement- Do not be physically lazy though. Just like your brain, your body needs to move efficiently and move fast at the same time, to be a good dueller, for the same reasons. You can practise this by exercising your muscles, especially focussing on flexibility, swiftness and agility. Your body also needs to listen to your brain very quickly, therefore, you must make your body learn quick responses and quick action on cue. You need to make duelling moves your reflexes, to be good at duelling. That comes to you with practice.

Think Out of the Box- Do not only think about the standard duelling spells or techniques you know. Think about other solutions that can solve a problem faster or better or be a faster or better method of offense or defense. I am talking unconventional means of duelling, even ones which you have never heard about being used in duelling, if it suits your purpose, such as shattering glass, transfiguring objects, mobilizing objects and sending them your opponent's way(just like the famous witch Minerva McGonagall used Piertotum Locomotor in the Battle of Hogwarts), using plants like Devil's Snare or pulling out the Mandrake while the opponent's ears are uncovered (with your own earmuffs on of course), pushing your opponent right within range of a full-grown Whomping Willow, using Stinksap to distract your opponent, causing a funny distraction using humor and attacking the opponent while they are distracted, burning up their shelter using the Fire-Making Charm, using potions for offense or defense, letting dark magical creatures loose towards your opponent with caution or using dark artefacts sparingly for the greater good in a good way("for the greater good" and "in a good way" largely comes down to your ethics and morals). Use potions, herbology, magical creatures, transfiguration, runes, alchemy or anything else you can think of. Sometimes, even magic which is not meant for duelling can help you in duelling, and offensive means can be used defensively and defensive means can be used offensively, so think out of the box. As long as you are not breaking the wizarding laws of your country or going against your own ethics and morals or endangering your own life or your loved ones(a stray spell hit the late famous Albus Dumbledore's sister while he was duelling the famous Gellert Grindelwald along with his brother, Aberforth Dumbledore, and killed her, and such happenings are likely to haunt you forever or cause other harm just like Albus Dumbledore was haunted by his past forever) or property or the same of others, you are good to go with anything you come up with, for duelling.

Know What is at Stake- Know how important winning the duel is to you. Know what you could lose from losing and what you could gain from winning the duel. That will keep you energized when required, if a lot, such as life or property is at stake and that will also help you determine when to leave, how long and upto what point to continue the duel, how much magical energy to spend and how much effort and action to expend for the duel and help you know when to give up and when to continue.

Know Your Limits- Overexertion or unnecessary harm to others are not very good outcomes. You could die or permanently lose your magical ability from overexertion or magical exhaustion. Know how far you are able to go and how much of an ordeal your brain and mind and magical energy are able to take. From this, you will know when to give up and when to continue and when to stop. Do not push yourself beyond your limits, if it is not worth it.

Keep a Sound Mind- Keeping a sound mind is necessary. Don't drive yourself to insanity. What I mean is that if you act in the heat of moment and do not carefully consider the repercussions of your actions before acting, or act too distractedly or stay lost in your thoughts or caught up in your head or suffer from crippling fear or depression, you could hinder your own duelling ability and put a lot at stake. You could end up performing actions that you later regret or harming people or property unnecessarily. Come from a place of calm and composure, so that you can think properly and clearly, and ethically and efficiently and quickly. Also, overexertion and magical exhaustion can lead you to not have a sound mind, so be careful of these.

Know Your Abilities, that is Your Strengths and Your Pitfalls- Don't overestimate yourself and don't underestimate yourself. Overestimating yourself can create a false sense of security which can backfire and quickly be your downfall and underestimating yourself can shake your confidence and thus hinder your performance, both negatively affecting your proficiency in duelling.

Do Not Underestimate Your Opponent- This closely relates to overestimating yourself or your ability to overcome your opponent, which can be harmful as I mentioned above. Know that your opponent can be much more than it appears, to prevent yourself from being in for a rude shock later.

Stay Alert and Active- Stay alert so that you don't drift away from the duel and lose your stream of thought or your physical agility and so that you don't get distracted and don't give your opponent a chance for an unannounced strike or accidentally reveal an opening or chink in your physical or magical armour and stay active to keep both your brain and physical muscles alert and ready to swiftly function at the slightest notice.

Know Some Good Duelling Stances and Good Duelling Positions- Duelling stances and duelling positions are there for a reason. Know the reason behind each, so that you know better when to use them. They help you gain an advantage over your opponent or provide defense against known means of duelling. Research on them and learn how and why they are used, then use them to your advantage.

Constantly Research and Seek New Knowledge- Duelling methods are flexible and duelling means and methods, just like the Dark Arts, are mutable, and constantly changing and evolving. Like I said earlier, know more to do more. Never stop learning no matter how proficient in duelling or powerful you have become. If you don't do this, something you didn't know about may become your downfall one day.

Use Psychology- (Or reverse psychology, you know what that is, right? If not, please research as it is very useful)Distract your opponent, make them underestimate you to harm them like I discussed above about how overestimation or underestimation can harm someone, show a different personality, or use means to slow them down, and gain an edge on them. Use any psychological tricks or tactics that you find useful.

Trust your instincts and your gut- Trust your gut and your basic instincts as they know best what to do. If something feels bad in your gut, it probably is. If you sense danger with your basic instincts, you probably are in it. If it seems too good to be true, it probably is. This approach is very helpful to prevent yourself from getting caught off-guard by anything, including your opponent during duels.

Be Curious- Curiosity is your friend, because if your opponent is doing something fishy under your nose or behind your back, your curiosity and your proficiency in smelling out fishy things, will help you catch the fish, instead of killing the cat. That will prevent you from getting caught off-guard when your opponent puts their fishy plan into action.

Be Willing to Adapt- Adapt your methods according to the kind of opponent you are facing and the circumstances. One example of this is that if you are a formal dueller, you must be willing to bend the rules if you are duelling a dark wizard or fighting for your life or for the lives of others, as these are dire circumstances and the dark wizard or any opponent who intends to cause you harm will not follow the rules of engagement and that may hurt you or your loved ones or others or property if you do not answer with tit for tat. Therefore, be kind but flexible. Do not let people walk over you, even in duelling.

Keep a Constant Eye on Your Opponent and Know What They are Doing Even When You are not Duelling- This relates to actively guessing your opponent's next moves and planning ahead and preparing for an appropriate answer to your opponent's upcoming move. Keeping an eye on your opponent even when not duelling helps to guess your opponent's upcoming moves, purely by observation.

Know the Repercussions of Your Actions- Before using a means of duelling, know what using it could do to you or your loved ones or to others or to property. No matter how heated up you are or what is at stake, try to avoid means of duelling that harm the aforesaid or cause inconveniences, unnecessarily.

Know When to Use Offense and When to Use Defense- You can use an offensive stance or a defensive stance in duelling or use offensive or defensive methods. It is important to assess the situation, circumstances, your abilities and limits, and the same of your opponent to know what will work better in your favor-offense or defense. You can primarily attack or primarily defend or use a combination of both and change that according to the aforesaid factors. That will also change according to your goals and what you want from the duel and what is at stake, for example when you want to back out quickly and escape or do not want to cause much harm to your opponent due to ethical or whatever reasons, you may need to use defense, but if you want to completely defeat the other party or cause harm for some reason, you may need to be more offensive and use offensive methods to finish the duel quickly or use a combination of both.  Both offense and defense are valid and equally respected duelling techniques. Use both as and when required. 

Practise, Practise and Practise- Do not forget to practise. The golden rule of anything you want to learn and be good at is practise, practise and practise. Practise as much as you can and whenever and wherever you can. Practise in any way possible. Engage in duelling for practice and go out and fight dark creatures for practice, because danger when it comes will come unannounced and you will have to be prepared to face that.