Exercises.

I think the best exercise would be constant practise, but I am not an expert in matters of the piano.
 
http://en.wikipedia.org/wiki/The_Virtuoso_Pianist_In_60_Exercises

Read a bit about this book. I know wiki is unreliable, but I can vouch for this, because my teacher has me training with the book. It's got LOADS of exercises in it that focus on building each finger's strength individually, and I feel that it helps.

You could also get a grip-strengthener when you can't get to a piano and squeeze that all the time. When your hands are stronger, playing piano usually becomes easier.
 
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