Here's an idea from Kegan McCabe's (Omega Longbows) site:
Working Man's Limb-Stabilizers
These limb-stabilizers work to dampen vibration in the limb (removing hand shock and noise). They're also very cheap and easy to add or remove.
Any good sized bike inner tube, cut into fourths with the nozzel removed, can be wrapped around the limbs about four inches from the end of the riser. You do not need to glue or tape them directly to the limbs, but rather just wrap or tape the ends to itself. No marring the finish, no permanent affixment. You can also add another set (the rest of the inner tube) right at the end of the riser for even more mass, making your longbow sit quietly in hand like a heavier balanced recurve!