Put your bra on -- the right way -- by slipping your arms into the straps so the cups are in front of you. Pull the cups down over your girls. And use a mirror, or do it blind, and fit the hooks into their corresponding eye loops at the back of your bra's band.
Adjust the girls with your hands. Bend over at your waist and slip your hand in each cup and move your breast so your nipple is smack dab in the middle of the cup. Does any of your flesh spill over the cups? If so, you need to go up at least a cup size. If your boobs don't fill out the cups, go down a size.
Adjust your bra straps. Notice how there's a little plastic adjuster on your strap? On one side of the adjuster are two pieces of fabric lying on top of each other; on the other side is only one piece of fabric. To make your strap shorter, pull the adjuster toward the single side of the adjuster. Your straps should be smaller if they feel loose or are falling down.
Now pay attention to the band of your bra. Is it lying underneath your shoulder blades, in the middle of your back? Because it should be. That's part of the strap adjustment. Once you get it in place, how does the band feel on your torso? If it's loose at all, undo it and move the hooks one set of eye loops to the left to make it smaller.
When you're trying a bra on at the store, do some lunges and some arm lifts or twirly twists. Bend over. The more you move in your bra, the better you'll be able to adjust it to fit you just right.