Make sure that the Wii Balance Board is placed properly in front of the TV: The blue power light should be facing away from the TV. If the Wii Balance Board is facing the wrong way, your movements will be reversed.
Check the battery life of the batteries in the Wii Balance Board. If the battery level is low, try a fresh set of batteries. Nintendo recommends the use of Alkaline batteries.
Check to make sure nothing is pressing against the bottom of the Wii Balance Board. If you do find anything pressing against the bottom (such as carpet), try using the foot extensions.