Capitol Hill is perfectly safe- it's almost all yuppies.
Share a rowhouse with people. I don't really think commuting from the burbs is a great idea. To get seriously cheaper than Capitol Hill, you have to go so far outside the city that commuting would be a major drag. Besides, even the far-out places get expensive if you are close to a metro. I find the best screener for Craigslist is writing ability. People who fill their emails with text-speak and make numerous grammatical errors are much more likely to be sketchy.
I live in this area, it will likely cost you more to live outside DC and commute in/out of the city (unless you're far away from the metro like nealric said, I know people who live 1 mi from the metro in arlington and pay $600 a month). Housing in popular suburbs like clarendon, bethesda, etc close to the metro runs around the same cost as sharing a row house on the hill. Plus, nothing beats a 5 minute walk to work... (and ~15 to GULC)
I found my house on craigslist and had no problems, i also know people who have had luck just walking around the hill streets and calling the phone numbers on "for rent" signs outside of houses.