Bend your snowboard before riding. Attach yourself to both bindings and hop back and forth from the nose, which is the front of the board, to the tail, the back of the board. This is your snowboard's longitudinal flex. Do this on the snow so you don't scuff your board or get wax from the bottom of your board on anything.
Break in your snowboard. The more you ride, the more flexible the board becomes. Longer, firmer boards are less flexible than shorter boards because they are for heavier people, but you can break them in just the same and end up with similar flexibility as a shorter board if you board enough.
Buy a softer board. If you're having a hard time maneuvering or feel you are going too fast, it is best to buy a snowboard made specifically for flexibility. Longer, harder boards will only break in so much and are made for free riding, which are designed for half-pipes and to float well on powder, rather than freestyling.