Excellent Question, @Jeroen. I have had the same problem for a long time. I am not a Wing Chun practitioner, but this has haunted me all my life throughout my Martial Arts study on all stand-up Martial Arts styles, but specially Aikido and BJJ. The interesting thing is that it was via Aikido and BJJ (and by that I mean, non-striking Martial Art) that I found a way to relax and use the same concept onto striking-based Martial Arts.
I spent some time thinking about it and started realising and experiencing some new improvements that I would love to share.
From the point of view more of a stand-up/striking Martial Art - Wing Chun - as far as I can see, I noticed three basic components associated with that (the third one below is variable; it will become clear in a minute):
Confidence, Breathing, ‘Chain Strike’
Confidence, is the ever-present element. By having more confidence on your skills and technique, you will be definitely more relaxed. In turn, the more relaxed you become, the more are you able to absorb, learn and more confident you become. In other words, you need confidence to relax and gain more confidence; and the more confident you become, the more relaxed you become. It is as if Confidence were the universe encapsulating the moment of a fight, or sparring, or training. It floats in the air like a thin cloud.
Breathing,. Deepening into the concept, breathing is the fuel, the drive behind your physical and mental preparation. It is quite difficult for me to put this into words, but when you are, say, throwing a punch, if your breathing is tense, your punch will lack speed, flexibility and more importantly, control and accuracy. If you were to practice in slow motion, for example, get to the stance or position you are training, pull all the air you can into your lungs and hold your breath for 5 or 7 seconds (important - Don’t hold your breath, as in shutting your throat, but simply stop breathing: there is a big difference between them. If you ‘lock’ your breath, you are creating tension, which goes against the whole point here). When you simply stop breathing for a few seconds, you will feel your brain oxygenated. This should bring a slight feeling of peace, then relax, by releasing the air again and as you release the air, focusing on trying to feel if your muscles are relaxing. I often imagine myself melting on a hot pavement as I relax. Once you do this for say 10 times, start adding a punch while you are releasing the air. This brings me to my ‘chain strike’.
‘Chain Strike’ - You know when someone uses a chain to launch it against a wall as if it were a whip? Noticed how relaxed and fluid the chain is while it is travelling? It goes in this liquid, fluid and relaxed motion until it hits the wall, but when it hits, it does so pretty hard. Well, I base my striking on this concept. How? By adding this third element to the aforementioned ones. I keep in mind the importance of developing confidence; I use the breathing-relaxation technique and attach it to the delivery of the ‘Chain Strike’. So again, I breath in, simply hold my breath without locking it, release trying to ‘melt’, but now, as I am releasing the air and relaxing my muscles, I also throw a very slow punch: I try to synchronise the duration of the punch with the duration of the air being released; so my aim is to hit the target right at the time when I have exhaled all the air left in my lungs. (Do it in slow motion; it is a lot harder to keep control when you do things slower). Why do I insist on this peculiarity? Because, when you are doing this at high power/speed, it this explosion of air that adds all the other benefits of a punch, such as speed, control and accuracy.
If Confidence was the ever-present atmosphere around your training/fighting environment, breathing was the fuel to help you deliver your actual attack. So you have the environment, the drive or fuel and the actual action, i.e., run, jump, punch, kick, defend, etc. In this case I used an attack, the ‘Chain Strike’ analogy which is why this third element has a variable nature: being relaxed is present on all elements of your game.