That's been talked about before here...not sure how you would search for it, though.
Anyway, a few years back,
Bass Player mag ran a good exercise.
Basically-
Pinky @the 12th fret, G-string
Ring finger @the 11th fret, G-string
Middle finger @the 10th fret, G-string
Index finger @the 9th fret, G-string
Attempt moving ONLY your INDEX finger to the D-string((9th fret)
Then move ONLY your middle finger to the D-string(10th fret)
NOTE: All the while, the other fingers remain fixed to the fretboard. Do not allow them to lift up.
USE YOUR OTHER HAND TO HOLD 'EM DOWN!
After all the fingers are on the D-string, move them, 1-at-time, to the A-string & so on.
Eventually, you can begin to move only 2 fingers while keeping the others fixed.
Example: Put your fingers back on the G-string as mentioned above.
Now move/toggle only the middle & ring...move them ever so slightly up & down, up & down.
Again, the index & pinky should NOT move or even wiggle.
Again, use your other hand for assistance.
Come up with the other permutations/alternatives(e.g. ring & pinky; index & middle; etc)
It's definitely hard at first...it takes concentration & practice. Afterr a week or so, you should see a vast improvement.
I usually do this sorta thing in front of the NHL games on ESPN.
