I used to live in a one-bedroom apartment by myself and now live in a two-bedroom with a roommate; however, when my roommate is gone for the night I do the same things I used to do when I was living alone. I make sure all of the blinds are closed (don't want anyone looking in), leave the light where the dining table is on (just so they never know if I'm awake or not), and ALWAYS keep my apartment door locked. Keeping it locked though is something I do whether the roommate is there or not. I've actually locked maintanence guys in on a regular basis because I'm just so used to automatically locking the door once it's closed.
Also, when Brendan was living in on-campus apartments last year I'd have a bit of a walk from his place to visitor parking (which was in a dirt lot right next to the woods) so when I'd have to walk to my car late at night I'd literally FORCE him to walk with me and then drive him the short distance back to his place. There was no way I was walking alone out there!
Along the same lines, if I have to walk pretty much anywhere by myself late at night I always have my cell phone open and to my face as if I'm on the phone. Not many people would want to attack someone who's already on the phone!
I think those are my main habits.