Balayage. I have dark blond (mousy, I've heard) and I've been getting highlights to make it light blond in the summer and a little darker in winter. I've tried many different techniques including, single process tints, cap frosts, foils, cellophanes, ombre, and balayage. Because we're so lucky to start with a really good base color that lightens easily without brassiness, balayage can look really natural and can easily be lightened or darkened, or, high and low lights without obvious root regrowth.
Don't go to an inexperienced hairdresser. Find a colorist if you're in a major city or if not, ask for an appointment with the owner or mnager of the shop. Don't let them talk you into making an appointment with someone who has openings. Find some balayage pics of the color you want to show the stylist.