Just saying "practice" really isn't enough. You have to be doing exercises that work on stamina.
Here's a great one I learned from Dave Weiner (guitarist for Steve Vai):
Alternating between your index and middle fingers on your left hand, play this:
----------------1-2------
-----------1-2-----------
------1-2----------------
-1-2---------------------
Then slide up 1 fret and come back down, starting with your middle finger on your left hand:
-3-2---------------------
-----3-2-----------------
---------3-2-------------
--------------3-2--------
then slide up another fret, and repeat, starting on your index finger:
----------------3-4------
-----------3-4-----------
------3-4----------------
-3-4---------------------
Go up and down the entire neck a few times, quickly but cleanly, using a metronome to build up your speed.
Then, switch to alternating your middle and ring fingers on your left hand, instead of your index and middle, and go up & down the neck a few times, and then switch to your ring and pinky. Then, alternate your index & ring, index & pinky, and middle & pinky.
Then, use all four fingers, like this (index, middle, ring, pinky):
------------------------------1-2-3-4----
---------------------1-2-3-4-------------
-----------1-2-3-4-----------------------
-1-2-3-4---------------------------------
then slide up one fret and come back down, reversing the finger order (to pinky, ring, middle, index):
-5-4-3-2----------------------------------
----------5-4-3-2-------------------------
-------------------5-4-3-2----------------
-----------------------------5-4-3-2------
and so on, up and down the neck a few times.
This is what I do to warm up before I play. If you do this at, say 1/16th notes at 100 bpm for 3 minutes without stopping at all, half a dozen times a day, you'll have excellent stamina in no time.
For an extra workout, get a cheap bass with high action (or raise the action on your bass temporarily) and practice these exercises on that.
Edit: Aha! I found a video of him showing the same exercise:
http://youtube.com/watch?v=3sto4sLaXrQ