How can I naturally increase breast size by one size?

Breast size up. The only way to do this without assistance, would be to gain weight. This, however, would be a very imprecise solution, as you cannot predict how much size increase you would get with any given body weight increase. Fat grafting, currently quite popular, enables you to have some fat from your own body transferred to the breast. This is an option for some patients.
Breast enlargement. Of course garments/bras can be used. There are several other techniques to enlarge the breast with breast implants being the most popular. Either saline or silicone implants can be used. Other methods include external expanders (brava device), or fat tranfer via grafts or flaps. A variety of hormones can be used but are generally used only for male to female transgender treatment.
Surgery. Breast augmentation with saline or silicone implants is one of the most common operations performed by plastic surgeons. No natural enhancement, exercise or other "non-surgical" breast augmentation cannot provide the same predictable, consistent results.
Natural breast aug. The term "natural" is often used when procedures do not introduce artificial or foreign objects into your body. Natural breast augmentation, the transfer of stem cell-augmented fat to your breast, falls under this definition. All the cells are your, the breasts feel entirely natural, no scars, and you get lipo to make your body contours even better!