A few people have mentioned it obliquely, but I'll come out and say it: Flat out refuse to accept a long-term guest.

It's one thing if a friend is visiting from out of town for a weekend (heck even a couple of weeks), as long as you know they're headed back again.

It's another thing entirely if they're moving to your area and "need a place to stay until they get settled". Same thing goes for new friends in your new town. Don't let people crash at your place.

This is really hard advice to give or to follow, because you'll feel like a monster when someone asks you for a place to stay and you say no. However, if you don't set a hard line on this, you're going to wind up with a roommate who doesn't pay the rent.

If it's ambiguous, then set a hard exit before letting them in "Of course you can crash at my place for the weekend. I can totally put you up until the 15th!". Setting clear expectations makes it easier to point out that they've overstayed their welcome.