To strengthen my fretting hand I regularly use a fingering exercise that Victor Wooten shows on his Bass Day DVD. Its based around the major and minor scales :
Major
G-----------------------------
D----------0-2-3-2-0---------
A----0-1-3----------3-1-0----
E-1-3---------------------3-1
Minor
G-----------------------------
D-------------1-3-1-----------
A-------1-3-4------4-3-1------
E-1-3-4------------------4-3-1
Try to do these by having your index finger on the 1st fret and using your pinky to fret the 4th fret of the E string. But move the scales up a half step each time, so once you've done it staring on fret 1, move the pattern onto fret 2, and then fret 3 etc etc.
Once you get as high as possible, start from the highest point possible on the A String and apply the same patterns and work your way back down fret by fret.
I've been doing this for the past 2 weeks and it's really strengthened my left hand, particularly the pinky.
Hope these are of some use and happy playing
Andrew