It's never easy to say "I don't like you," but sometimes that's the best thing you can do. If you're not interested in someone, be nice and let them know the next day. Don't string it out and don't make it a big deal. Chances are they have a few other irons in the fire, too. This line has worked for me (trademark pending): "It was great to meet you but I'm not sure we're compatible, romantically speaking. Good luck."
Chances are you'll know when you first meet them whether you'll want to venture out on a second date. If you're not sure, and you want to give them another chance, refer back to Tip #8. You may want to pull the cord on your escape chute eventually.