I've read and heard about many different methods.
1. Drinking two to four glasses of milk (soy or animal-produced) per day
2. Massage breasts for ten to twenty minutes with oil (vegetable or olive), conditioner, or lotion. You can also use friction from rubbing your hands together.
3. Birth control pills. If you are old enough to take them and if your parent will allow you to, hormones in birth control are proven to enlarge breasts.
4. Eliminate junk foods from your diet and eat healthy. Drink lots of water and exercise.
5. Push ups are a great pectoral exercise to increase bust size.
6. Wear a push up bra or use silicone inserts. Although this doesn't actually enlarge your breasts, it creates the illusion that they are larger. Don't stuff your bra with tissue, toilet paper, etc. It's so obvious.
7. Take an herbal supplement. Some include fenugreek, wild yam, fennel, or a combination of all three. Check with your doctor before taking these supplements as the herbs may conflict with medications you are taking.
8. Let nature take its course. You probably aren't finished with puberty so don't worry about it and let your boobs grow naturally.
