Start exercising regularly. It will not only tone up your body, you will feel much better about yourself. You don't have to do anything special; just regular physical activity will be great for you considering that you are not overweight.
Invest in a new wardrobe. Shop for clothes that make you look more lively and cheerful. Choose colors that go well with your skin and hair colour. Do not forget the accessories.
Buy high-quality lingerie. Do not be too worried if it is too risqué. You will feel great when you wear it.
Spend more time in front of the mirror. Dress up in your favourite outfits and admire your body in front of the mirror. Just because you have small front, it does not mean that you should not look at yourself in the mirror. Notice, in particular, those assets of your body that stand out, whether it is your hair or eyes or rear.
Read inspirational books to develop your self-esteem.
Stay away from people who tease you or try to convince you that a woman's self-esteem is to be determined by her size.
Finally, smile. Smile for being a woman.
