A warm-up is different than practice. Practice is to improve, maintain, or fine-tune a technique. Few players improve unless they do lots of this. However, the focus of this tip is on warming up.
Warm-up is to groove your shots before playing matches. (You also warm up before a practice session, but that's more to loosen up before serious drilling.) How can you improve your warm-up?
Don't go to the table cold. Do some easy jogging and perhaps some shadow practice to warm up the muscles.
Especially for a tournament, try to arrange in advance to warm up with someone you are used to practicing with. The partner you choose should be a relatively steady player - you can't groove your shots against someone who hits each shot erratically. Your partner should also be reliable – it doesn't help if he doesn't show up or shows up late when all the tables are taken.
You should decide in advance what you need to warm up. Make sure to warm up every major aspect of your game! That means forehands and backhands, crosscourt and down-the-line, looping, blocking, footwork, and so on. Don't forget pushing! Many players will push erratically in a match because they didn't warm it up. Even serves should be warmed up, especially deep ones that can be tricky to pull off if not warmed up first. For most techniques, 2-3 minutes might be enough to warm it up, but take whatever time is needed.
A really good warmup, covering everything, takes at least 45 minutes or more, though you can probably get most of it done in 30 minutes. The less time you have, the more you'll have to cut corners. For club play, you might not be able to get all this in unless you come early. Many players simply warm up the basics for maybe 5-10 minutes, and then warm up the rest by playing matches. You can do that, but it might not optimize your play.
Finish the warmup by playing actual points. You might start off with common rally types. For example, you might have one person serve backspin, the receiver pushes long, the server loops, and play out the point. At the end, play regular points. Many players leave out this part - and the first time they play regular points is in a real match. Not the best way to prepare!
How do you know if you've had a good warmup? You'll feel ready to play. If you don't, then you didn't get a good warmup. And the nice thing is that, cumulatively, these warm-ups not only prepare you for a match, but they also help you improve as you groove your shots.