This could be a very short Tip, since the mentality in a match and in practice should be the same. So here's the short version: think of a time when you played GREAT. It could be in a tournament or a practice match, or even a practice session. The key is that you played great, and want to play like that all the time. Now think about your mentality when you were playing great, i.e. were "in the zone." You were probably playing almost mindlessly, other than tactical thinking. In fact, you probably were more like a spectator just watching yourself react mindlessly and almost flawlessly. THAT is the mentality you want both when you practice and when you play a match.
So how do you do it? Many think that they only need to reach this state of being in the zone when they are playing serious matches. But that's like saying you only need to have a good forehand in serious matches, and so can goof off when practicing and in less serious matches. Just as you need to develop your forehand in practice and in practice matches, you need to develop this ability to get into the zone by practicing it - and that means doing so whenever you play until it becomes second nature. It is only then that you'll be able to turn it on at will in big matches, because it will then, in fact, be second nature, and just like your other best techniques. Think of getting in the zone as no different than hitting a good forehand. Both are techniques that take practice, and if you want them to be ready in serious matches, you need to practice them . . . in practice.
How do you get into the zone? Basically you have to just let go and let your subconscious take control. When someone loops to you and you block, you don't consciously set the racket angle; your subconscious does this, as well as just about everything else you do when rallying. When you practice, you are basically training the subconscious to play, i.e. setting muscle memory so that it'll react properly in a match. It's not a static thing; if your opponent does something different that you haven't practiced against, your subconscious will learn and adjust. The key in all cases is to let it do so, since the subconscious is quicker and more accurate than anything you can do consciously. If your conscious interferes by trying to guide your shots, your game will deteriorate. Other than thinking tactics - which the subconscious will pick up on and follow - think of yourself as a spectator and just admire your play. Heck, I sometimes feel guilty when I'm playing well since I feel like I'm not really doing anything other than watching as that subconscious part of me takes control. (But remember - the conscious mind has its role as the tactical leader that the subconscious will follow if you allow it. And yet, even there the subconscious will end up doing most of your tactical work since it will learn to automatically do the right tactical thing. It's smarter than you think!)
Putting aside physical limitations, there's no reason why a player can't be at his best essentially all the time - it's all in the head. When players realize this, as most do after they've played many years, their game goes up dramatically. Do you want to wait many years or learn this now?