My doctor commented on my lack of stretch marks during my first pregnancy. I told her I was putting body butter on my belly. She said the only thing that has been shown to actually work is vitamin E and good genes.
I don't know about the "good genes" part, but when I got home I checked the label on my body butter. Sure enough, it had vitamin E in it! I had started using it daily in the shower the minute my tummy bulged, and I didn't get one stretch mark.
My body butter is from 100% Pure. They make tons of different scents; just find the one you like. They're a "natural" company. I will admit the stuff is greasy--just like butter. So I put it on as my final step in the shower and rinse off the excess. My tummy still has a good layer on it after I dry off.
Don't spend good money on those marketed pregnancy creams. I would just look for something with vitamin E, and be religious about using it. Also, when your belly starts itching, it's a sign that it's dry. So slather on some cream when that happens. Good luck!