This is just in anticipation, right?
I'd suggest relaxing a little (and try not to assume that all your flatmates will be broke plebs who steal food). Sure, maybe you'll get someone who takes some milk sometimes, but in all seriousness I doubt you'll be with anyone who's horrendous. Sure, you'll hear a lot of stories about people like that, but it's not a reality that it happens to everyone. Just be positive about who you're housed with, firstly.
However, on the off chance that it does turn out to be a horror story, I'd listen to the advice of these lovely people above me.