The exercise I use to get more strength on my pinky finger is this:
G|-4p1-4p1-4p2-4p2-4p3-4p3-4p3-4p3-4p2-4p2-4p1-4p1
D|-----------------------------------------------------
A|-----------------------------------------------------
E|-----------------------------------------------------
Then move the exercise to the D string, then A, then E then A again, then D and last G.
It's important that you play long notes and get the pulloff right.
Don't just lift your pinky finger off the string, pull it off so that you get the pull-off right.
This is a very wearing exercise, but try to push yourself to the limit.
When I first began using this I could not get to the A string before I was spent, but now I can run the whole exercise on my 5-string.
Use this exercise everytime you warm-up and you should really start to notice a stronger pinky after a week or so.
It's also important that you play your basslines using the one-finger-per-fret rule.
For example, if you've got a bassline that goes like this(this is the bass intro in For Whom The Bell Tolls by Metallica btw):
The bracketed part under the tab is how you should finger it.
1=Index, 2=Middle, 3=Ring and 4=Pinky
G|-19-18-17-16-----------19--------
D|-----------------17-16------------
A|----------------------------17-16-
E|-----------------------------------
[---4--3--2--1------2--1--4---2--1--]
As you can see there's one finger per fret and you only move your fingers, not your hand.
If you just start applying this rule to your basslines and warm-up with the little pull-off exercise above, you'll start to build up some strength in your lefthand ring and pinky finger.
This worked for me, but of course, your milage may vary.
