Renting out a room to a stranger

I'd echo the point made about interviewing potential roommates. Probably really obvious, but you're going to have to make it clear to the person moving in what you will and won't tolerate, so far as hours for bringing guests ("bad people") over. I've been in instances where roommates have brought over rowdy crowds after 3 am, without regard for the fact that I had 8am class the next day.

Also, find out whether he/she's a night owl or not. If you have thin walls and a person who uses stereo sound for music, don't let them hinder your sleep by telling them beforehand to use headphones, etc.

Jam it back in, in the dark.