Well there are plenty of workouts that have worked for me that I've checked here on this site. I guess that it's not always focused on the arm but the entire body for a pitcher to have good stamina as well as good mechanics. Our coach has told us just like all the others that it starts below in the legs, comes up through the hips, to the torso, and through the arms. Squats, lunges, tricep curls (bench dips), latteral raises are all good body building mechanics for most pitchers.
Bench dips are exactly what they sound like. Just find a bench, put both hands on the bench behind your back with your heels flat on the ground. Relax your muscles to let yourself dip down to the point where your gleuteous almost touches the ground and then push back up. Also, if you have any weights, put a stable weight or two on your stomach to have some extra resistance.
I'm sure that you've heard of the others and if not, check the workouts portion on this website for the latteral raises. The squats and lunges I'm sure you've heard of before as well since I figure you're older than me.
Basics are a good thing too from what we've been taught. Find a bench or a table you don't really use and a towel. Spin the towel around until it is nice and tight and fold that in half length wise. Start out from the windup and when you come through as if you were throwing a ball, smack the table with the tip of the towel. If you're too close, move back and if you're too far, move it.