What is the best way to thank your GM?
Let me count the ways. As I mostly GM I think feel qualified to answer this one. The best is to actually show up and be engaged. Missing multiple weeks for weak reasons or showing up and not paying attention will drive a DM crazy. We put in a lot of time and effort and all we are looking for is that the players have a good time. Now, I will admit there are times when the DM drops the ball and runs a lousy game but if it is a half-way decent game night show it during the game.
-Buy them some supplements. I wish there was a gift registry for DMs and rpg books. This also works for replacing their old broken books.
-Bring the food/snacks. My Thursday night group is awesome. They rotate the players with each one buying dinner for everyone each week. As the DM/host I don't have to pay.
-Coming up with extra character stuff like pictures of your character and their contacts.
-Don't argue or be belligerent if things don't go your way. Trust your DM.