It happens to everyone. You are getting along fine with a girl, and then all of the sudden, dead air. You've got nothing to say, and she is either too nervous, or you haven't hooked her well enough for her to fill the void. I have come up with a new technique that has worked marvelously for saving many of these scenarios.
First, look her in the eyes for a a couple of seconds (this will often get her to volunteer a conversation, so that you don't even have to do anymore work. It will also help if you are so nervous that you haven't really given her a chance to start a conversation thread before).
Now, burst into a little chuckle, then turn away slightly.
If she ignores you, you might as well leave, she isn't interested.
Most women, even the ones who were only just slightly interested, are going to ask you what you were thinking about. Tell her that she just reminded you of a really funny story, but say it in a kind of off-handed manner. She is going to want to hear the story. Tease her with it. Tell her, that she wouldn't get it, or that you will tell her another time. When she begs you to tell her, tell her that she has to tell you a crazy/funny story from her past first.
This will get her to tell you some event from her life which will start a new conversation flow. This technique will almost always generate a great story from her past which you can use to really get to know her. It will break through the artificial barriers. Once she finishes, just continue threads of conversation based on her story. You could also try to tie her story into an actual experience, funny or not, that you have had in the past.
Generally, your "funny" story won't come up again, but if it does, just kind of dismiss it, and continue the conversation where you left off, or actually tell a prepared story from your life.