Based on nearly seven years of experience, I feel I can give good advice on this . . . So, to quote Shania Twain, "let's go, girls . . ."
1) Act like you are the only person who lives there
If I'm the only person who lives there, I can be as much of a douche as I want . . .
2) Your music can NEVER be loud enough.
You THINK it's as loud as you can get it? I'm sure you can find supersonic speakers or something that will make it louder. Go on - I know you want to...
3) Dishes WILL wash themselves
Just make sure they are as DIRTY AS POSSIBLE.
4) Door slamming is ESSENTIAL!
Otherise how do people know you're in the flat???
5) Take other people's things.
It might be something they don't notice readily - like a bit of their milk - or something they WILL NOTICE - like a big chunk of their expensive (gift!) Molton Brown bubble bath . . . but whatever it is, you need to make sure you don't put it back where you got it from so the person KNOWS you stole it! (Otherwise where's the fun!)
6) Never buy toilet paper. Because other people will ALWAYS Buy it if you don't. We're all human after all. We ALL have to wipe our asses.
7) Make sure you time your toilet visits with every point you know your flatmate MIGHT POSSIBLY want to go in there. (It's nice to be somewhere that's in demand, right?)
Any other contributions???