A practice match is just that - a practice match. The problem is that many only get the second part - "match" - and forget about that first part - "practice."
This doesn't mean you don't take a practice match seriously or don't give it 100%. It's not a goof-off match; it's a practice match. But how should you approach a practice match differently than a tournament match?
To start with, you should treat most practice matches almost the same as a tournament match. That means playing to win. But there are differences. Winning may no longer be top priority. On the other hand, playing well should always be a priority in practice - even if you don't win.
In practice matches, if the goal is to improve, then this is your chance to practice whatever it is you need to improve. For example, if you're a blocker who is trying to develop his loop, practice matches are an opportunity to do that. Decide how you want to play in the future, and strive to play that way in practice. Your goal is to play well and to improve.
But not all opponents are the same. Some are weaker; some are about your own level, and some are stronger. How should you approach these matches?
Playing weaker players is a blessing. Now you can really practice your shots! Here's the chance for a blocker to work on his serve & loop; the looper to work on his blocking; and so on. Pick out something you need to work on, and go for it. Need to develop a counterloop? Let your opponent loop first and try to counterloop. Have trouble looping a heavy backspin? The weaker player might not have a good attack, but if he can push heavy, you have your practice. And so on. Against weaker players, practice your weaknesses, practice against their strengths, or go all the way and pit your weaknesses against their strengths. Or perhaps practice your own strengths, and see if you can completely dominate the match, if that's what needs work.
Against players your own level, find a balance. Here's where you incorporate the things you developed playing against weaker players. When there are no tournaments or other important matches coming up, try to improve and add on to your game. If there are important matches coming up, focus more on playing to win, while picking and choosing what new techniques you can now use. Use the match to practice your normal game.
Against stronger players, here's your chance to push your own strengths to a higher level. Improving isn't just about working on your weaknesses; it's also about making your strengths dominating. Do you have a good loop? Now's your chance to see if you can dominate a stronger player with it. Do you play fast? Now's your chance to see if you can play at the pace of the stronger player. And so on.
Let's suppose one of these "practice" matches is close, and it's near the end of a game. What do you do? You could try winning with the shot or shots you are working on. Better still, this is your chance to practice learning how to win. So instead, play to win using whatever tactics are best. Practice isn't always about practicing a shot; it's also about practicing tactics. Learning to win, and what tactics to use to win, are huge practice.
Let me re-emphasize that you should give 100% in practice matches; otherwise, you aren't being fair to your partner, and you are wasting your own time. But use the time intelligently, and optimize the development of your game. Play to win, but play to win using the shots you want to use for winning.