Have you done any measurements since you started working out? As far as I know, stretch marks appear when the tissue can't keep up with the expansion and stretch put on the skin from either muscle growth or weight gain. The best way that I would think to prevent them from growing is to slow down the rate of muscle growth, if that's in fact what's happening. I know there are products out there that claim to get rid of stretch marks, but from what I understand, it's more of the 'massaging' of the stretch marks that causes them to diminish.