HundredPushups.com is a great exercise site. They have sister sites for sit-ups, squats, and pull-ups. Each site provides a simple program that takes about 30 minutes a week, and promises that over six weeks you can dramatically improve your fitness.
The secret? A graduated sequence of repetitions in five sets. Each sequence is repeated three times (say, Monday, Wednesday, and Friday).
HundredPushups.com can give your workouts a highly effective structure. The goal-setting is done for you. All you need to supply is a bit of will-power. Go check it out!
(Thanks for the tip, Sean!)