The easiest way is to gain weight or finish growing (if this is your case). If not, the following solutions will help you either way:
A word to the wise -- there is no magic pill. Marketers make money from our insecurites and pop culture media has sent messages to men and women that they are somehow deficient in their appearance to the opposite sex.
You can strengthen and tone your deltoids (muscles across your chest) to appear more toned and built up. I suggest you ask an expert at a gym and get an actual demonstration.
There are exercises that help your posture. Keeping your shoulders back and chest lifted gives the appearance of confidence and beauty.
If you are a mature woman or not finished growing, remember that a woman has a hormone called estrogen that gives fat pads in her breasts, to fill them out. You must daily eat healthy omega 3 fatty acids in your foods that stimulate the pads in the right places and keep your weight in check. The best palce to get this is in salmon, fax seed, sesame seed, olive oil and nuts. Eat this with raw green veggies and berries to support your muscle tone and quality muscle tissue and skin cells (which breasts need). This fact is known by marketers who to sell it to you in a cream or pill, so save your money. You become what you eat.
Cosmetically, if you choose to show cleavage, you can aply bronzing powder with a large rounded brush, above the chest and in the cleavage. This makes a big difference.
Also, a proper fitting underwire bra, even without padding, helps. You must be fitted by an expert in the women's lingerie department who takes your measurements.
A healthy self-esteem can help many of smaller chested women feel that the size we have is sexy enough and it projects! Wear clothes that fit properly, make you feel beautiful and womanly. Put your shoulders back, honey. Don't hide your chest.
If you are trying to become more attractive to the opposite sex, please remember that at a deeper level, in a quality relationship, a good man will notice you as a person and love you along with the package God put you in!