I'd echo what previous posters said and stay away from Craigslist - some real creepers hang out there. Contact someone in your department and see if they have a list of other students looking for roommates, because surely you're not the only one. Also check if your school's housing office maintains a list of students from other departments who might be looking for roommates. By only looking at other grad students you'll be drawing from a much better pool than the general Craigslist population.
As far as finding compatible people, have a phone/Skype chat with them. In my experience, communication is important, and you really need to address issues early before they become major conflicts or sources of resentment. Nothing else will make your life quite as miserable as going home to a hostile situation.
Lastly, look for people who have similar lifestyles to you. You'll want someone who has similar tolerance for noise and mess, someone who has about the same workload (it sucks slaving away while your roommate has fun), and someone who has similar beliefs about sex, drugs, rock&roll, etc.