The best thing I know of is to throw, throw, throw. Do lots of throwing, every day---not pitching, just throwing, 20 to 30 minutes at a time---I can think of no better way than that. Lots of throwing, as in playing catch; this will build up and maintain arm strength as nothing else can. And do a couple of full bullpens a week, with a catcher behind the plate and a good pitching coach standing by and observing what you're doing. As for building up speed, velocity, whatever you want to call it, I recommend something I learned a long time ago---THE SECRET. I learned this by watching how the Yankees' Big Three rotation did it; they drove off the lower half of the body, using the legs, the hips and the torso in one continuous motion, getting the whole body involved, thus generating more power behind their pitches and taking a lot of pressure off the arm and the shoulder so they could throw harder and faster with less effort. There are some drills and exercises you can use to your advantage; the best one is the so-called "Hershiser" drill which requires no special equipment, just a fence or a wall. Long toss---up to a point---can also be helpful.
And---work fast and throw strikes. 8)