Well i have read somewhere in this board and also made that experience myself that playing for a long time just straight easy but relatively fast stuff (for example straight sixteens @ 100 or 120 bpm) until your right hand is really exhausted works well of improving your stamina (of course no exaggerating here, don't play until your hand hurts for and hour afterwards

)
playing with or without tension in your right hand is an important decision, really strong exhaustion afflicting your playing should only occur if you play with tension, if you want to get a Steve Harris-like hammer plucking style, thats the way to go
many bass players prefer a lighter style as you can see above, I think if you practice enough Stamina you can play light and hard as you want which improves your variety of dynamics, many players say that this is a great advantage of playing fingerstyle
but there are numerous other ways of plucking the ****, for example John Entwistle used a typewriter-like technique as described above from irishnyo
I also think playing some Slap n' Pop really improves your coordination and overall finger and hand dexterity, especially for the thumb
I must repeat these are only several ways some might do many other stuff ^^