I've done the craigslist thing before, and it's like shooting fish in a barrel. Every ad on there is some chump who has no idea how to write or how to convey personality. They just put up three or four lines that say,
"Hey, I'm a fun loving, nice guy who loves sports. GO PATS!!! I've never really done anything like this before, but it's just so hard to meet a good woman these days lol!!! So I don't really know what I'm suppoed to put here, but I do like to treat my woman right. I just want a good woman who wants to be good to her man. So if you think you might be into that, shoot me a line or something."
I'm serious, go look at your local craigslist. It's nothing but thirty variations of that every day. The guys have no idea how to convey anything about their personalities through writing. If you reread the few lines I just wrote, you'll notice I didn't say ANYTHING in it. And that's what most of these guys do. They list a few vague, general personality traits that are in no way unique or even descriptive of their personality (for instance, saying you're fun loving isn't saying anything, because everyone loves fun. Have you ever met a fun hating person?)
They don't say anything about themselves, and they don't say anything about the type of girl they want to meet. If you have half a brain you can easily write a much better ad than these guys. And if you have the best ad, you'll get the most responses.
When I've posted in the past I just write a lot about myself in ways that convey my personality. She wants to read and get an idea of who you are, and what you're looking for. The next day I check my e-mail and there's about 10 girls talking about how they "never respond to these things, but you just seem so funny and sweet and honest, well I figured I'd give it a shot."
It's great for if you want to get some practice in, or if you just haven't been laid in a good minute and need to get some. These girls tend to be on the desperate side. Craigslist is the only online thing I've ever done, and I picked it mainly because it really plays off of how competetive girls are amongst one another. You can ask them to qualify for you, and they will.