Down in front: Is the center of the bra lying flat against your breastbone? A gap there means the cup or band size is a little too small -- try the next one up. Also, the band should always rest entirely under your breasts. If it's on top of them at all, adjust the shoulder straps, or go for a larger size (a 34 to a 36, for instance).
Keep em covered: Are your breasts spilling out over the top or creeping out from under the bottom of the cups? First adjust your breasts with your hands so they're sitting in the cups. Still got more breast than bra? Go for a larger cup size or a different style.
Turn and look: When you move around (arms up, arms down, spin around), does the band ride up? That's most common problem women have and it means the band or cup size is too big. Try fastening it on a tighter hook and trying a different, smaller size, bra. Which is more secure without causing back spillage?
Give the straps a break: Can you slip a finger easily underneath each strap? You should be able to! Most of a bra's support should come from the band, not the straps. If your band is too big, your straps will end up taking the weight, causing red marks and general discomfort: No thanks! Try a smaller band size.
Put it to the test: The dressing room is one thing -- what's the bra really do for you? Slip on a T-shirt: If you see extra fabric or wrinkly cups, try sizing down. If you see the line of the bra and your breast popping out, try sizing up. Then pick up your kid and take a lap around the store to make sure the bra doesn't pull, poke or dig when you move. Finally, take another look to make sure everything's still in place. It is? You have a winner!