Collect a non-trivial amount upfront, and pay that back to the user if he actually completes a Goal
This I guess, would increase the commitment from the user if he knows that his money is at stake. Also it would feel like a real cash reward when he accomplishes a goal.
If he doesn't complete a goal as per plan, or if he even "derailed from the road", a chunk of the stake money goes to Beeminder.
Also, you guys could collect a flat fee of say 5 USD per goal, even if the user completes the entire goal. I would be happy to pay you that because your system allows me to be a better person. And you get to make money too. Win-Win :D
For more fun (or adrenalin), you can make the user choose his own stake amount.