try a gripmaster,or this exercise, which piano players use.
rest all four fingers on a table,then lift first finger and put down again,do this with all four fingers,keeping one finger up while the other three are down.
do this first with index , then the middle , then the ring , then the pinky.
dadgbd is also a good tuning,lots of two fingerd chords for d and g,but you can still think in standard for the inside four if you want to.