How It Works
Make a pledge to keep your inbox (or any other Gmail label) within some limit or to whittle down the number of messages. Choose only Read messages if you don't want new messages coming in to count against you. Be sure what you're committing to! Beeminder will charge you money if you don't stay on track!
Every day, make sure you reduce your number of messages by the amount Beeminder tells you. (More details!)
Keep following Beeminder's Yellow Brick Road to gradually whittle down your backlog. Once you get to Zero (or whatever you want to beemind your inbox down to), adjust your Yellow Brick Road to make sure you don't spiral back out of control.