[Matthew_Armeni]

Well you first have to determine whether they are unbalanced, bent, or both. If it does it the whole time the you need to check other things. You should remove your rear hub carrier with the axle still in it and tire mounted to it. Hold the hub in your hand allowing the tire to spin freely, and spin it. When the tire comes to rest add some playdoh, clay, or hotglue to the inside of the wheel at the top (because the heavy side is at the bottom, that's why the tire came rest the way it did). Spin again and add or remove the weight, this should only take 3-4 spins to get the tire/wheel balanced, any more and you're being picky. Then remove the tire and repeat to your other 3.
Another thing to observe while you're spinning tires is the trueness of the spin, you shouldn't be able to see any wobble or wiggle side to side. If you do, look for cracks in your wheel, especially around the hex. If it's just warped you may be able to bend them back a little, but it's ussually just a temporary fix.