Depending on how much you hold a grudge...

Everyone who picked on you at high school, everyone who picked on you at primary school, the disapproving family members who told you to go get a real job, the insecure best friend who constantly undermined your confidence, and generally anyone who ever doubted your ability to finish what you started in style.

At least, that's my motivation. It may not be very healthy, but it gets the job done.