1. Train, train, train!

The easiest thing you can do in order to earn XP and level up to different belts quicker is train. As you progress through the game, you will continue to unlock new training items. Each of those typically has subsets inside of it as well. For instance, you can have many trampolines to work with. Lower level ones replenish faster so don't take them for granted. Whenever you can use them for training, make sure you do so you accumulate as much XP as you can, as fast as you can.