Positive visualization, everyday, with increasing regularity as a fight approaches. Visualize yourself boxing/grappling/fighting a perfect fight, slipping and countering seamlessly, with flowing defense, having your opponent hurt and fighting on his back foot. Imagine the walk to the ring/cage/whatever, the stare down, all of it, and when the time finally comes for the fight to happen, it will feel like you've been there before and are just reliving the experience.