Ohh good question :) I also really want to know the same thing.
Right now I use a Shea Body butter cream (super, super thick cream) from Bath and Body Works. It is so, SO freaking moisturizing. I put it on before going to bed and in the morning my belly is still moist. I have no idea how well it prevents
stretch marks, but it sure makes the skin soft and I am guessing elastic as well.