Don't be shy!

Go on up and talk to the most famous person there, or any speaker who was particularly interesting.

Particularly if they are just standing around in a group or, even better, doing nothing at all.

They *love* hearing that you liked their talk, and will be even more pleased by an intelligent question about any aspect of it that wasn't 100% clear -- That tells them what to expand in the next conference they give that same talk.