If you can't afford a personal trainer and you're too busy to spend an hour at the gym, then Daily Workouts may be helpful for you. Its 5-30 minute routes are great for targeting bodyparts. This app has separate workouts for abs, arms, butt, and more. Video tutorials demonstrate how each workout is done. And a countdown timer lets you know when to switch exercises. Workouts can be customized to maximize effectiveness.
It's up to you to stay motivated. But I can tell you from firsthand experience, this app helps!The full version of this app is $3.99. It's compatible with iPhone, iPod touch, and iPad, and requires iOS 4.3 or later.