[siracguy]

One approach I've found useful when trying to meet any of the speakers at a conference (which I assume includes the FAMOUS GUY you refer to): find the speaker in some other setting at the conference, such as when everyone picks up a boxed lunch and sits at random tables, or people are standing around outside the conference room. At that point, the speaker is just another person hanging out (unless the person is REALLY FAMOUS, and getting mobbed all the time). I once met the Fed CTO at a Starbucks just outside the conf center.

Right after a speech is the WORST time to try to strike up a conversation with a speaker, as some of the other commenters have pointed out - at that point, they're usually surrounded by a group of people handing them business cards and talking at them; yet that's when most people try to meet them.

Getting a friend to introduce you to someone he or she knows is usually the best way; most people will be happy to perform an introduction if you just ask. In the worst case, they can just tell you that they're not comfortable doing it.