I use an oil called 'Mother's Special Blend' and it has coconut oil, almond oil, Vitamin E, and grapeseed oil all together. I started using at right before 6 months, and I rub it on my belly after each shower. So far so good--I'm at 31 weeks and basically no stretch markes yet even though my tummy got really big.
My facialist also told me that Fresh's Rice Bran Oil is really good for stretch marks, but it's quite pricey. Smells awesome though.
I think I remember that you should look for a product with Vitamin E---I think because it helps to build collagen?