Relax, be yourself. Worst that can happen is, nothing – you'll not "lose" anything, you'll be in exactly the position you are now. Don't sweat it too much and you'll ace it, go in like a bundle of nerves and panic and a) you'll say something stupid and b) no-one wants to hire a stressbunny.
Think of answers for stock questions. "Tell us your best feature… ok, now tell us your worst feature…" is a HR favourite. (Unprepared people can be shockingly frank, I once heard a candidate reply to that with 'well, I have anger management issues, I left my last job after a fist-fight with my boss. Er, he started it.')
If you don't know something, be resourceful. Eg, on a technical question, "well, I don't know the answer, but I could work it out from the manual / ask a colleague / check their website / I once had a similar situation where I did this…"
Don't lie. Really. You'll be found out.
Don't put stupid crap in a public place and think no-one will look. (I once interviewed a candidate for a sysadmin role, did a bit of legwork off his CV beforehand and came across his personal website where he was boasting about what a great hacker he was and what systems he'd cracked and burned… right… )