You know what a cup of coffee or spot of tea can do for an afternoon slump. Not surprisingly, it does the exact same for your workout. "Caffeine simply makes us feel better so that we can soldier on with our activities for longer before our productivity, concentration, or motivation wane," says Louise Burke, one of the world's leading researchers on how caffeine affects physical performance. Drink coffee, slurp a caffeinated gel, or have flat soda (carbonation can cause stomach upset), and your mental focus will also double, says Burke, giving you the self-discipline to hit the gym…and stay there.