A bra that fits well can do more than just help you look good—it can also make you feel healthier. "A bra that's too big, too small or just doesn't fit right can cause back pain, irritate your skin and generally make you feel uncomfortable," says Jené Luciani, author of The Bra Book. Follow these buying tips from Luciani to find the perfect bra, no matter what your shape or size.

1. Measure before you go. Your bra size can change from year to year, so make sure you're up to date. Changes in weight and hormonal fluctuations (pregnancy, breastfeeding, menopause) can both affect size. Use a flexible cloth measuring tape to measure two areas: directly below your breasts around your rib cage, and around the fullest part of your breasts. The first number determines your band size, the second your cup size—each inch above your band size equals one cup size. So if your band measurement is 32 inches and your cup 35 inches, you'd be a 32C. You can also ask a salesperson at a lingerie or department store to do it for you. But it's a good idea to have some baseline numbers before you go.

2. Try it on the right way. Lean about halfway forward, swoop and scoop your breast tissue into the cups, then fasten the back. Stand up and adjust the straps to ensure that all of your breast tissue is where it should be.

3. Watch for key fit concerns. On the no-no list: Boob spillage over the top of the cups (too small). Gaping in the cups (too big). Straps digging into your shoulders or sliding down. Any kind of pinching in the backband, or rolls of skin that bulge beyond the edge. You should be able to comfortably fasten a bra on the second or third hook—if only the last hook will do, it's too small. The bridge of the bra (centered between the cups) and the underwire around the sides should sit completely flush with the skin.

4. Buy enough to last. Depending on how often it's washed and worn, the average bra can last from 6 months to a year without getting stretched out. Investing in two or three styles at once means you'll have plenty of choices in the months ahead.