I'm going to let you in on a little secret... Speed/dexterity/endurance has nothing to do with the strength of your fingers/hands. It's all about being as relaxed as you can be! Make sure your action is nice and low, too
Start by playing scales. Pay attention to any tension you might have in your hands/fingers. Remember to keep the fingers loosey goosey! Try looking at your hands in the mirror while you play bass; They should look totally relaxed. Note: If you don't focus on getting rid of tension problems, you're well on your way to getting carpal tunnel or arthritis. This takes practice, but after a while, your fingers won't cramp up so much because you have cured any tension! On a lot of beginner videos, the players will tell you about the one finger per fret rule. I believe in this rule very strongly if you are talking about the frets AFTER the 5th fret. For me, the one finger per fret rule in the first 5 frets seems ridiculous. The stretching causes a lot of tension in my hands (I have small hands) so I ignore certain aspects of the rule for the sake of getting rid of tension. Most rules have their exceptions
Now, make sure that you're fretting as lightly as possible. Creating an unnecessary struggle for your fingers to play will make you tired very quickly. It's like stomping your foot on the ground every time you take a step; Is that logical? For your fretting hand, try to fret a note as lightly as you can before you hear buzzing. You want to make every single note like this. When you are able to play with the lightest touch possible, you can fly all over that fretboard at will!
As for your plucking hand, only pluck as hard as you have to. If you dig in too hard, your hands will cramp up and lose their integrity! Remember: not every note has to be accented! Yes, it's important to use articulation as a tool to express yourself, but don't let lack of technique cripple your playing.
These tools should definitely help you with your speed/dexterity/endurance, and your overall playing. As Anthony Wellington and Victor Wooten say, "Don't try harder, try
easier!"
Anyone care to add?