From what I've heard from others, the best way to think of "pulling up" is to imagine trying to hit your handlebars with your kneecaps. I combine that with imagining that I'm trying to kick my own buttocks with the heels of my feet, and "kicking out" at the top of the stroke. I think the real trick to pedaling sustainably, is not to put too much effort into any part of the pedaling. Rather, concentrate on keeping cadence without applying noticeable force at any part of the stroke, as if you were pedaling your feet in the air.

I have to do it this way myself, because if I concentrate on the pedal stroke, I always suffer from "cadence creep" where I suddenly find that my pedals are going 'round faster than my feet can keep up with. I end up in too high a gear ratio, and end up "mashing" just to maintain my pace. If I concentrate on my cadence instead, everything else sorts itself out.