Watch the Pick Tricks online guitar lesson by Jeff Scheetz from Rock Tricks
For starters you should have your name on your pick. This is free advertising, helps you remember who you are, and of course when your buddy pockets one "by mistake" it is easy to identify and convict. Other than that, you can use your pick to get all kinds of weird noises and sounds from your guitar that otherwise wouldn't be easy or perhaps even wise to attempt.
And of course you can use it to toss in the air when you are bored with the changes on the song you are playing. And then at the end of the show you can throw it out into the crowd. Best of all, you can usually judge the effectiveness of your performance on whether someone keeps it or just throws it back at you.