You don't want to be rude and interrupt when someone's telling you a story they've already told you, but you also don't want to have to listen to the whole thing again. Especially if it's a long one. My friend Paul and I agreed that we were repeating things often enough that it was time to make up a gesture. The one we came up with was to point at our ear and do a little tapping motion. Paired with a smirk, it works great.
I have a feeling it might work even with people who haven't seen it before. At the very least, there's a good chance they'll ask you what you're doing, and you can explain that they've told you the story before. Try it! Let me know how it works.