Here's a tip from Boy's Guide to Girls: 30 Pointers You Won't Get From Your Parents or Friends:
Q: How many times should I ask a girl out before giving up?
A: Every situation is different and it takes experience to judge when enough is enough. As a general rule, though, if she responds with, “I’d like to, but I can’t make it this time,” give her the benefit of the doubt and ask again later. If she says the same thing a second time, it’s probably her way of saying “no.”
If she gives you a clear “no,” or a good reason, like “I have a boyfriend,” then one time is enough. It’s tempting to go back later with an “Are you sure you won’t change your mind?” but, the best thing to do is gracefully accept the “no” and move on.
+Boy's Guide to Girls