The back of the upper am, the triceps, is a problem area for women because it's a prime place for storing fat...hence the dreaded bingo wings. It doesn't matter if you're slim or you could do with losing a few pounds: flabby arms are a female curse!
Targeted exercise can reduce the circumference of your arms and sculpt them, firming your bingo wings.
Good sports: Boxing, tennis, swimming, climbing, sailing.
Every little helps! Use your arms as often as you can (when you're doing the housework, for example, or carrying heavy things - shopping bags can be surprisingly heavy!).