You should try some hand exercise equipment like Vari-Grip tool, Flexbar hand exerciser, Gripmaster etc.
Doing pushups with fingers also make your fingers strong and it also increase your body power.
Playing Guitar makes your finger strong. Typing on Keyboard or in cellphone (sms typing) also good for fingers.
If you really want to make your fingers strong then you should also make your body strong by doing grip exercises like pull ups, finger push ups etc.