I actually laminated a Markdown Cheatsheet a while back as I have a plugin on my website that allows for Markdown use on a Wordpress blog. I tend to use iA Writer for the Markdown and then post it straight from the app to my Wordpress blog which makes it easy and I always have a local copy on all my devices. So I've managed to memorise some of it. But I post so infrequently that it's not really stuck in my head yet to any great extent.