Here are some pictures of the the empty tennis can training device for the development of a curveball. I call it a curveCAN, but it's just an empty tennis can, which can be purchased at Wal-mart for $1.79. (Just take the three tennis balls out of the can, of course.)
These first two pictures show how the can rotates up-and-over, end-over-end. Look closely and you'll see me pointing with my index finger. I often like to think about "shooting the target." In other words, that index finger, for training purposes, points directly where you're going to throw the curveCAN.
In the next two pictures, you can progress into throwing with your normal breaking ball arm path. Again, the curveCAN is gripped at the base with your preferred curveball grip, and the can is spun end-over-end with 12-6 rotation. Release the curveCAN, just like the pitch itself, out in front so the first rotation is downward, not humping upward.
A pitcher can work on this during the off-season three to four times a week at 25-30 spins a day without any added stresses on the throwing arm, which is why I like the curveCAN so much. Granted, it's basic, but for learning the pitch, it was highly effective in my development.