Sound Money Tips has some ideas for protecting your deposit on rented accommodation – a few years too late for me, as I got stung for part of my deposit when I left my last student house (ok, I had used blu-tack to stick my posters up, which supposedly meant the wall had to be re-plastered, but I was still pretty miffed).
Main point – take photos of the state of the place when you arrive (with the availability of digital cameras now there’s no excuse not to take some snaps). If you’ve got nothing to hide, walk through the place with the landlord when you leave.