I found the planet waves unit useful in this way - I could use time when I couldn't play, specifically while driving, both for strengthening generally but most importantly for isolating the movement of the ring and pinky fingers on the left hand. When I started with it, I could not get those fingers to move individually (like I could on my right hand). After several months, I have been able to get them to work better independently, which has definitely helped with actual playing and use of the pinky.