In my advanced drawing class, we are focusing on multi-figural compostions, requiring me to draw in public a lot. I've been having trouble with this, because my teacher wants us to draw at the Sports and Rec Center. It's really hard to draw people doing sports or exercising, because they are constantly moving. I find that I'm only able to get a quick glance at what they're doing, like a 1 second snapshot, and have to furiously scribble down what I can remember. Thus, I've found myself mostly drawing from memory, and many of my drawings are crap. Even a 10 second pose on posemaniacs is easier.
Does anyone have any tips on how to go about this? I know that the obvious answer is to practice practice practice. But apart from that, does anyone have any practical tips on how to do this?