I've actually had very good luck with dating sites, personally. It's pretty simple to find interesting girls. You generally have to look for girls who state that they're just looking for friends, as I know a lot of girls who use dating sites to find local, like-minded people. Becoming friends with a girl like that is a wonderful way to begin a relationship.
Also, when approaching someone from a dating site (Via e-mail or IM), don't be too pushy. Don't bring up sex, dating, or relationships at all. Instead, try to find people who have your interests, and talk about those. Ask questions, offer a tad about yourself, then repeat. If she has no questions to respond to, she may not respond at all. Don't e-mail her daily. E-mailing her every-other day is best, so that you don't seem too obsessive.
Use. Correct. Grammar. Girls really appreciate good grammar. Use it as much as possible. Try not to use too many "LOL"s. Remember, the most worth-while girls are very intelligent and usually not very attractive. Very attractive girls tend to be stuck-up, idiots, or down-right sluts.
Try not to mention your ideal girlfriend or the type of relationship you're looking for on your profile. Simply list your interests, and what you're doing with your life. Try to keep it to about three paragraphs. Don't write a novel for a profile, but also try to not be too vague.
Oh, and I suggest "Plenty of Fish" and "JustSayHi" in addition to the sites you've mentioned. Good luck!