Put a small see saw style wooble board on a low plinth and place a mirror infront of pt. Sit behind them on the plinth and control lateral movement. You can further develop ex by placing a ball under the hand and encouraging the pt to roll it sideways along the bed as they move. You can also progress by raising the plinth so the pt's feet are no longer in contact with the floor -ie reduce base of support- quite a tough one! Encourage slow controlled movement and make sure they maintain good upright posture throughout. Then progress to sitting on large physio ball placed on the floor.