Increase Fretting Hand Strength
When I started playing guitar I found I had difficulty holding down the E and A strings with my third and fourth (pinkie) fingers enough to get the strings to ring correctly, also hammer-ons were not as loud using my third and fourth fingers. To increase my finger strength I got a soft rubber ball. Holding it in the palm of my fretting hand I practised squeezing it one finger at a time. This exercise also helps with isolating the movement of your fingers. The great thing about this exercise is you can do it anywhere. Dont go too crazy though, you dont want RSI or finger cramps!