If you're a reliable customer who pays her bills on time, you should be able to get your credit card company to lower your interest rate by simply calling and asking about it. You have nothing to lose by asking, and the credit card company has a lot to lose (you!) if you decide that one of their competitors is offering a better interest rate.
If you'd like to make the call but feel uneasy about what to say, consider the script CBC News gave to shoppers during their mall experiment, which led to six of 10 shoppers getting their interest rates reduced (three of them even had their rates cut in half!). Here's the spiel: "I think I've been a good customer. I'd like to stay with you, but I really want you to lower the rate on my card. Can you help me?" If the customer service rep refuses, ask for a supervisor and repeat the script.
The customer service rep will likely offer you a new rate first but that doesn't mean you should accept it, so aim low! And don't hang up until you've gotten a good deal.