I tried match.com and udate.com a few years back. (Although I have met most of my best boyfriends at random bars I don't usually go to on nights when I didn't want to be there and was definitely NOT trying to meet anyone. Really wanted to go home, actually - this is the truth.)
I have friends that are on plentyoffish now. I think for any of these sites the same rules apply:
1. Be yourself - it is too hard to keep up the pretense of being someone else (more physically fit, less/more talkative, art/sports lover, etc)
2. If something seems off/wierd (fetish, negative, etc) - run! Believe in your instincts.
3. Give it time but don't waste time. If someone seems nice - give them a chance for a few dates. On the other hand, if you don't like them - let them go - making room in your life for someone you do like.
4. Talk/meet soon & publicly -some guys just seem to want to email forever (maybe they are married or just don't really want to meet someone?) - anyone can write an email (I could be copying this right now for all you know) - talk to them so you can see the rhythm of your conversation & meet them (In a public place for safety!) so you can see if there is anything there.
5. Be open to surprises... I have met the best people and had the best times when I wasn't expecting it. (On the other hand, prom and New Year's - big planned "fun" events - have often been disasters!) So let life bring you great things at surprising times in unexpected packages.
I have lots more thoughts on this but will let others weigh in...