Lemon juice works wonders on not only acne scars but stretch marks as well. Simply rub some juice on the area and let it sit for at least 10 minutes before rinsing off with water.
You can also mix lemon juice with baking soda for immediate visible results. Mix enough to make a paste, cover the affected area and wrap with plastic wrap for deep penetration. Leave on for 30 minutes to an hour. If done once a day for two weeks, your stretch marks will vanish.