"Diet and Exercise".
Really, it's the best way to reduce man breasts - unless, of course, it's a real medical condition other than just being a little jiggly up there.
If you don't have access to a gym, doing wide-grip push-ups and pull-ups (like someone else already mentioned) are great ways to build muscle up there. This will widen your chest and help turn your "man-boobs" into a hot, chisled (sp?) chest.
If you have access to a gym, there are many other workouts that help, as well. Dips, various benches, and a few exercises with some resistance cables really help. Oh, and the butterfly machine is great as well.
It's true that you can't really "spot" reduce where your fat is going to burn, but I do find I'm much more tone in areas that I do strength training (biceps, chest, calves) than in areas I oftentimes neglect (obligques, lower back, and my flabby ass).