Above all, do a lot of throwing---even if it's just playing catch for fifteen or twenty minutes a day.
Several days ago I watched Bob Costas' interview with Bob Feller on the MLB Network, and Feller astutely observed that a big problem with today's pitchers is they just don't throw enough. Whether it be just playing catch or doing a bullpen session, regular throwing is just about the best way to strengthen the arm. My pitching coach told me, "Throw every day", and I did that---not only to strengthen the arm but also to keep it loose and flexible and thus avoid injury. 8)