For me keeping my weight gain to a minimum didn't work. I lost over 30lbs in my first trimester with my son and gained a little over 20lbs back but I still have horrible stretch marks. I tried cocoa butter, vitamin E and who knows what else. Nothing worked. Every woman in my family that has had children have them. There's just no getting away from them. If you're going to get them you're going to get them.
Also, keeping your weight gain within managable boundries will help avoid stretch marks. They are caused by rapid growth that the skin cannot keep up with. Keeping your weight gain slow & steady will go a long way towards helping avoid them.
Vitamin E is actually the only thing that has been proven to help the elasticity of the skin. And afterwards I've heard amazing things about maderma, the one specifically for stretch marks. Just make sure to exfoliate in the shower and lotion up afterwards. Hopefully it helps out!!
Honestly, nothing will work. Stretch marks are genetic and if you're going to have them, it'll happen no matter what you do.