High-intensity interval training (HIIT) is an effective way to improve fitness rapidly. It is an unpleasant way to train, but it works. Caffeine and creatine monohydrate increase exercise performance and recovery from HIIT.
A study from the National Taiwan Normal University showed that caffeine (400 mgs) improved performance during high-intensity interval training in active men following five days of creatine loading (20 grams per day). The study showed that consuming caffeine prior to intense exercise enhances the effect of creatine monohydrate on performance. (European Journal of Applied Physiology, published online)