I would say the techniques that usually strengthen your left hand fingers are bending, legato and maybe certain types of vibrato.
Now, I don't think bending with your little finger is of any use, but I would certainly concentrate on trills. I mean, fluttering between your 1st and 4th fingers, 2nd and 4th, and even 3rd and 4th. Try to keep doing it for at least one full minute, maybe even two if you can. Concentrate on getting good notes, sounding clean and even, but do not put excessive force into it. You don't want to strain your tendons.
I remember Joe Satriani mentioning that in the past some of his students would go as far as wrapping rubber bands between their fingers in order to get them stronger. You can hurt yourself by doing that. Joe pointed out that you don't necessarily need more strength, you need good execution of a technique, which will spare you an unnecessary waste of energy. There are a couple of videos of Joe talking about this on youtube.
Then you could also practice vibrating notes with your pinky. But I found that for me, trills and legato playing get my left hand fingers stronger than anything else. I also remember Steve demonstrating trills with all fingers during his Guinness World Record lesson.
Have you ever checked out this little lesson?http://www.vai.com/pull-off-playing-incl-hammer-ons/