Answer:
The breast is made up of fatty tissue.
As such, a common and seemingly obvious answer would be to eat fatty foods, but the problem there is you have no control over where fat deposits itself on your body.
The best way to 'Increase' breast size is probably to condition yourself in a gym with light , high intensity toning exercises. This WILL NOT increase the size of your breasts, but it will tone the muscle under the breast up to make them appear larger.
Alternatively, you could try using padded or wired Bras. This of course only creates an illusion of larger breasts.
Pills/injections/supplements/electronic gadgets that promise larger breasts are ripoffs. Implants are an outside and 'non-natural' option.