I find that if you kind of anchor the uke against your rib cage by pressing gently in with the fleshy part of your strumming forearm, that arm will naturally be inclined to stay still, and therefore, force your wrist to take over the movement. Remember to stayed relaxed and keep the muscles in your wrist and strumming hand loose. Easier said than done, maybe...you could start by doing a relaxation exercise - just shake your wrists around a bit, like you're shaking the tension out of them, then, go and pick up your uke with a vision of yourself kicking back on Waikiki Beach...