I totally get what you mean. Honestly thats happened to me too. Usually I talk to my crush about him. What he does, what else he is interested in besides himself, and try to find common ground. Now this is for if you don't really know your crush that well. But if you do know her well enough and you have a past with her, ou can always bring up a past memory you two shared together or talk about something you both like or what she likes and you know a lot about.
Hope this helped :)