I pick something that is motivating for me at the moment. For example I might only let myself read a special book while working out on the stationary bike. When the book is done, I might set a new mini goal of sticking to 30 minutes of cardio 5 times a weeks for 3 weeks before I can get some cute new exercise pants. Mini goals help.
Mix it up using low or no cost motivators to keep from being bored.