How do I keep my breast augmentation private?

First of all, it should be mentioned that society has come a long way toward realizing the benefits of augmentation for a woman's life, and many women are surprised to find acceptance where they least expect it, among parents, children, and coworkers.
But let's assume that you do intend to keep very private about it - what can you do? First of all, let it be known that you are "working out" your chest muscles.

Then, go buy bras that are one size larger than you currently wear, and start wearing them all the time. Pad it out with inserts, fillers, pads, or else get one with built-in water or silicone inserts. Wear those for a month or so. If you are going up two sizes, after the first month buy the next size up, and repeat the process until for the month before operation, everybody has gotten used to seeing you at your desired size. Then after surgery, they will not notice as readily.
Another advantage of this method is that you yourself will get accustomed to the new size before operation, so you can be certain that you like that size.