Pad Mapper is a good site to use–you save time by not having to look up every place on Google maps.
Honestly though, looking for a place in DC is just…difficult. It took me probably at least 50 emails to Craigslist posters to find my current place. And the one I got was just by luck–I happened to respond to it 1 minute after it was posted, managed to be the first person to get there after work, and luckily they were looking to sign the first normal person who they found.
Many posters really aren’t going to respond, especially if you reply more than 24 hours after the ad was posted. As someone who has had to post on Craigslist for a roommate, I can attest that all of the advertizers are likely getting TONS of responses. I got more than 100 in 2 days, and then took the ad down because I couldn’t deal with more. You basically should just troll Craigslist and Pad Mapper constantly to up your odds. It sucks, but everyone’s gone through it, and it is worth it in the end!