A great workout for this is the spider exercise.

1. Place four fingers across the frets high up on the neck beyond the 12th fret. We start high up on the neck because the frets are closer together, and its easier to get our fingers to stay in place behind the frets.

2. Slowly start with the first finger and move it to the same fret, but a string above. The important thing here is to make sure that the other fingers do not move when you are transferring the finger to the next string.

3. Do the same for the next finger in line. At this point you should have your first two fingers on the D string and your last two still back on the A string. It is important to move the fingers very slowly. The slower you do it the harder it will be to keep the other fingers from moving.

4. Continue on and bring the other fingers into line four frets across on the D string. Continue on up to the G string in the same fashion.

5. Once on the G string, come back down, except this time start with your pinky.

6. After that, move your hand gradually down the neck and the frets will become further apart, making it harder to keep your fingers still and behind the frets.

This lesson is taken from the book "The Bass Players Companion"
by Chris Tarry.....