Pick one day a week to be your designated sprint day. This is the day where you run, bike, swim or whatever it is you like to do, harder and faster. You get on the road, treadmill or in the pool and either push it to the limit the entire workout or do intervals where you work hard for a certain distance, then back off and repeat this over and over again.
What is great about sprint days are your workouts are shorter in time and distance because you're working so hard. So, pick a day that you don't have a ton of time and use it as your sprint day. You will burn great calories and get stronger and faster.
P.S. Yesterday I did my run sprints while pushing my twins in the stroller. Talk about a workout!
Link to blog: http://thefitdivas.typepad.com/my_weblog/2009/03/sprint-day.html