Yes, here's a tip: RELAX.
Unless you're dating Prince Harry or a Gotti, there's no reason to be tied up in knots about meeting these people. They're just people, and probably worried that you won't like them, either. You have to relax, because the more nervous you are, the more likely you are to make a fool of yourself.
It's easy to make a good impression: just be yourself. Unless you're a raving moron or an obnoxious git (and you don't sound like either of those), you should be perfectly capable of making a good impression just by being yourself. If you try acting a certain way to impress, people will know it and will label you a big fat phony.
What exactly are you worried about? That they won't like you? Even if that happens, so what? You won't be the first girlfriend a guy's family doesn't like. You're dating him, not them.
Sure, it will be great if they like you, but it's not mandatory. I think they will, though, because you obviously care about this guy. And that is exactly what will endear you to them, the fact that you care about their son, brother, grandson, nephew, cousin. They want him to be with someone who truly loves him, and if you do, it will show. That's why all you have to do is be yourself. Pretty simple.
A glass of wine might not hurt, either.
Good luck. Let me know how it goes.