Use a moisturizer. A study comparing herbal moisturizers found that people who lotioned-up daily showed significant improvement in skin appearance and elasticity [2]. Think of it this way: if your skin has a lot of stretching to do, it's less likely to tear and create stretch marks if it's hydrated.
Use lotion on new stretch marks, or on areas where you're worried about getting them in the future. Though using lotion on old stretch marks may still improve their appearance, it's probably not as effective as using them on fresh ones.
If you're worried about the chemicals in over-the-counter moisturizers, make your own out of equal parts wheat germ oil, olive oil and aloe vera. The same study showed that this combination created the best DIY moisturizer [3].