If you don't practice very often, you know what it's like when you sit down to play and 20 minutes in your hands are getting tired. Even if you don't have that problem, strengthening your hands will give you better control and make playing easier. The easiest way to strengthen your hands is to get a hand exerciser.
There are a lot of different hand exercisers available, I've used two of them to strengthen my hands. Read more here: http://www.acousticfingerstylemusic.com/hand-and-finger-exercisers/.