Exercises for finger strength

You need endurance more than strength. First, try turning up louder than usual and focus on playing softly. Let the amp do the work. Then, woodshed like crazy. Practice at least an hour a day, every day. The more you use those muscles, the easier it will be. Lastly, and possibly most important, get a good bass instructor to look at your right hand technique to make sure you're not doing anything overly strenuous or possibly dangerous.

 

If it starts to hurt, stop.
Like Hobobob said, focus on endurance, not strength.
If it keeps hurting, stop.
Stretch your arms, wrists, and fingers before you start to play.
Warm up SLOOOWLY. Play major scales in whole notes at a slow tempo for a few minutes before trying anything else. Focus on tone, time, and correct playing technique. Build your chops slowly.
Practice slowly. If what you're playing is too complex to play at tempo, slow it down so that you can play it correctly without wearing your chops out. Then, gradually speed up - you'll learn to play the line correctly, and build your endurance at the same time (a metronome is your friend here).
And if it hurts at any point, stop.

 

consider your bass position, and how it effects the ebnd inyou wrist.
adjust your strap so your wrist can be closer to straight.
more bend in the wrist = more strain, tension, and effort to pluck / finger .
true for both hands.
+1 to starting slow.

 

I mentioned this in another post, but I drive all the time. I was having digital stamina problems playing sets of Iron Maiden covers. I started using my gearshift as a thumb rest and plucked away. I felt it helped my digital stamina and my 3 finger technique. Obviously every gear shifter size and location is different, but mine worked out pretty good for me.