Very nearly all women out there are wearing the wrong bra size. Cherry's experience is typical - by far the most common mistake is too small in the cups and too big in the band. Getting measured is the way to go. If you live near a relatively high-end department store, they usually have professionally trained fitters on staff. If not, a good lingerie boutique is your next best bet.
Wherever you go:
1. Call first and make an appointment. It's much more likely they will set you up with the person on staff that actually knows what she's doing.
2. If possible, go early in the day. The store will be less crowded and they'll be more likely to take the time and really help you.
3. Be sure to buy a couple new bras while you're there
Good luck!