A good way to strengthen the pinky is to use it in power chords. I only use my forefinger and pinky for rhythm and it definitly helped build fret stength when I started out. As Ren said, the rest is just practice - muscle memory has a lot to do with it too. So practice 123, 123, 123 and 1234, 1234, 1234 runs to get the fingers and tendons working.