Many people are falsely led to believe that exercise can increase breast size. This is definitely not true, however you can lift and firm your breasts with exercise.
Exercise is important for lift and to prevent sagging as we age. Exercise does not promote breast tissue growth, which is what leads to breast enlargement. Building the muscles underneath and around the breasts is all that exercise will do.
You really should do exercises for your chest, so that as you age you can maintain a "perky" look and firm breasts. But I will tell you again, breast exercises will not increase your bra size!
There are many exercises you can perform using light hand weights that are effective for firming the breasts. Push ups and planks (a pilates move) also strengthen the chest muscles.
The best regimen to increase your breast size and firm this area are natural breast pills, breast cream and exercise. If you combine all three of these methods you will achieve the best results possible.