In Denmark, a gym offers a membership program where you pay nothing as long as you show up at least once a week. But miss a week and you have to pay full price for the month. The psychology is brilliant. When you go every week, you feel great about yourself and the gym. But eventually you'll get busy and miss a week. You'll pay, but you'll blame yourself alone. Unlike the usual situation where you pay for a gym you're not going to, your instinct is not to cancel your membership; instead it's to redouble your commitment.
Src: KK's Cool Tools: How to use zero pricing