I used to use a magazine brush between stages, but discovered it was splaying the feed lips out, preventing the mags from dropping free. Now, unless the mag is truly full of dirt/sand/mud, I just hold the mag upside down, and work the follower up and down a half-dozen times. All of my mags have metal followers, and they act as a scraper, allowing all the junk to fall out the top or through the inspection holes. If the mag is truly full of stuff, I strip it and clean it out with a rag, before using it again.