I prefer older women. Not ALOT older, but a few years (26-30). I'm 25 so it's been my experience that most women younger than me are fickle. That may not be the case in 5-10 years, but it is now.
On the subject of eHarmony, though, I've been a member for a while now. It's not cheap, but I do like it. I've noticed ALOT of women are teachers, though. I'm thinking there must be some sort of marketing program or special advertising geared towards teachers.
I just started cleaning out my list last night. I had over 500 matches. Granted, I've been a member for quite a while already so I've had plenty of time to accumulate. I've narrowed it down to about 100. Today I'll probably narrow that down to 50 or less. That's still pretty good odds, though.