There is a book called "the virtuoso pianist"

i highly reccomend this book, it is aimed at improving stamina of the wrist, all fingers on both hands, trains finger independance and basically works both hands equally to be able to accomplish any task on a piano.

All that is reccomended is at least 30 mins to 1 hour of this book per week, and that will keep your hands/fingers equally strengthened. after playing for even 15-20 mins, and you go back to one of your songs, u feel like u can do anything.

good luck
Nick