Playing chromatics is one way of strengthening the little finger, but remember, that for 3/4 of the time you are playing with the other 3 fingers, so you're not homing in on the problem.
Isolate the little finger by inventing your own exercises and make sure that you use it in combination with all the other fingers. Here's some I made up especially for you, right now.
Ex. 1 (easy)
h p h p h p
-5-8-5---5-9-5---5-10-5 <--- this line shows fret
1 4 1 1 4 1 1 4 1 <--- this line shows fretting finger
Repeat 10000 times, using metronome. Make sure that every note in the exercise is the same length and the same volume (except the first note of each triplet, which you pick, so it will have a bit more attack).
Ex 2. (medium)
p p p p
-8-6-5--8-7-5--
4 2 1 4 3 1
Repeat 20000 times. Pick first note of each triplet.
Ex 3. (hard)
-7-8-5-------------------------------------------
---------6-8-5-----------------------------------
-----------------5-7-4---------------------------
-------------------------5-7-3-------------------
---------------------------------5-7-3-----------
------------------------------------------5-7-3--
3 4 1 2 4 1 2 4 1 2 4 1 2 4 1 2 4 1
h h p h h p h h p h h p h h p h h p
Repeat 30000 times. Make sure no other notes ring out, open strings, etc.
In this exercise, none of the notes are picked.
If you want *really* hard exercises, just let me know.