Ditto on just "throw, throw, throw". This will increase your off-hand strength AND your off-hand coordination. I've been throwing lefty for a few sets every time I throw. When I first started trying to throw lefty, I'd get 0 to 2 out of 10 sticks from 1/2 spin distance. After about a month, I'm up to 7 or 8 out of 10 at 1/2 spin and I'm up to 5 to 6 out of 10 at 1 spin distance. So it's definitely just more throwing that's needed to improve. An additional side benefit is that I find that to throw well as a lefty, I have to think more about my form and technique. This has bled over into improving my righty technique as well.