Use only sharp knives when cutting sushi rolls. Otherwise, they will really fall apart since you need to use a sawing motion to cut through it. With a very sharp knife, it will only take one slicing motion to cut through a sushi roll.
To slice, cut cleanly through the center of the roll. Then slice into thirds or quarters. Make sure to moisten the knife with water after each cut. You will only need to wet the blade and not the entire knife.
A wet blade will prevent the knife from crushing the sushi roll. It will also prevent the nori and rice from sticking to the blade.