Your mind may not be a muscle but it can really determine whether your workout is fulfilling or just another exercise in futility. Our think-tank of specialists reveal the mental strategies that can help you get more out of your routine—even when you feel like throwing in the towel. Visualization: Choose a set of images that inspire or make you feel good (a crowd roaring as you run around a track, skiing at your favorite resort) and replay them in your mind as you work out, says Steven Ungerleider, Ph.D., a member of the U.S. Olympic Committee's sports psychology group. Not only will…