I usually spend half an hour of my drawing period as warmup, just drawing 1-minute figure studies. That means everyday i manage to draw at least 30 human bodies simplified, usually in motion. It also helps getting the fluid motion in your drawings, because by the time you start doing your own work, you're already in "the zone". I really notice it in my own drawings when i haven't warmed up this way, since they tend to be stiffer.
I use http://reference.sketchdaily.net/en/ for models and timer.
