I'm going to try out some exercises that Old Woodchuck put together and posted at Old Woodchuck's Tabs. He used to call them "Mold a Mighty Pinky" but has since renamed them to "Old Woodchuck's Left Hand Exercises" when he expanded the exercises to include the other fingers.
The first exercise, which Old Woodchuck calls "the banjo-less finger exercise," is similar to one recommended to me by one of my cello-playing nephews, who called it by the catchier name of "finger pops." Here's his description:
Touch your pinkie and thumb together. Now your ring finger and thumb. Now your middle finger and thumb. Now your index finger and thumb. Repeat ad nauseum. That's finger pops.I'll try 'em both and see if I develop a pinky of steel.