I'm glad you brought this up. I've been going a while and, though I have been steadily improving, I've become somewhat concerned because I still haven't managed to work in any real 'pinky' action on the fret board. I guess that's one of the disadvantages of jumping right into learning songs and neglecting the technique. You have to be conscious of it and really work at it, especially when you jump 3 or 4 frets like in a walking blues bass line, which judging by your signature it seems that you like to play :) There's no secret I'm afraid other than practice. I used to only fret with three fingers and slide my ring finger up and down the fret board instead of stretching with my pinky but you lose control of where you're at unless you constantly look at the frets. The other thing I did was when I initially tried to fret with my pinky, I found that I was reinforcing it with my ring finger because, like I said, it wasn't strong enough. It just comes with practice and it pays off in the long run as your fretting hand doesn't have to move so much. Take a look at this guy playing some 12 bar blues riffs. Check out the pinky. https://youtu.be/XgJS82kOvl8?t=1m