What is the best way to prevent stretch marks in pregnancy?

Not much helps. Unfortunately, stretch marks are more a result of genetics than anything. There's nothing available that prevents stretch marks but keeping the skin moisturized will keep the skin from itching.
Genetics. Unfortunately our tendency to stretch marks is just bad luck. Stretch marks occur from rapid expansion of the skin and literally tearing of the collagen structure. Some people have more resistant tissue than others and will get more stretch marks. Topical salves and creams won't help. What helps is managing your weight gain, so exercise and eat healthy foods in reasonable quantities.
Normal weight gain. For some women, stretch marks are inevitable, simply due to their heredity, tissue health, nutrition, and other factors. But, in my experience, the most common cause for stretch marks relates back to the pregnancy course -- and excessive weight gain. So, for many women, preventing stretch marks may be a difficult, but identifiable, project...Maintain normal weight gains during pregnancy.
Not much to do. There really is not anything you can do. The tendency to develop stretch marks is genetic. Don't waste your money on creams, etc.