First, Good luck. There needs to be a winner in this area & I don't know of any candidates. I kind of feel that the secret sauce is something that needs to be discovered not invented. These are my thoughts (no particular order).
1. Most players are paid services. I think there is a good chance that whatever winner emerges will be a free service or at least free for 90% + of active users. The service is essentially better if it is free (network effects but more then that: You can take it lightly if it's free. It's serious business if it's paid) so any paid service would be in constant danger of a free clone emerging.
2. Let non-singles get involved. Encourage meddling. Set other people up. Introduce, create polls on who should be together. I dunno, something in that area. Maybe think along the lines of 14 yr olds instead of 30 somethings using a professional dating service. The former have fun. The latter do not.
3. Let people approach this from an angle. Just like in real life. Meeting in singles bars, via introductions, pickups etc. is only one way. Most people prefer meeting someone at work, meditation workshops or University. Dunno how you'de do that online, but it would be interesting if you could piggyback on ways people interact with each other online already. Facebook/Myspace is an obvious path. But maybe Digg, blogs, wiki editing, amazon reviews or even HN might provide a thread to pull on.
4. Be un-sleazy & you have an immediate edge (design looks right for this). That feeling that half the users are fake, that they inflate women or men's numbers... That's will be on users minds (good reasons too). All that damage needs to be mended.
5. Try to build in dynamism. However this is gonna work, I doubt anyone can predict the whole picture. Find some ways to let users find ways of dating online rather then handing them a system.
6. Let the kids show you how to do it. Maybe you should try & focus on getting dates for 14 yr olds. They might pave some sort of a path.
7. Maybe focus on the non market. There's a tendency to focus on those people that are having trouble. They're very goal oriented. There is a problem that they're going to fix. So many services are tailored around filling that whole. But there it might be easier to get earlier successes with those that will probably get a date this month anyway. You just fill the 'how'.