The way I learned was breaking it down, reassembling and doing functions checks with a mat similar to the one that Darklyte has in the OP. Eventually got rid of the mat and started timing myself. Then before I built my second AR from parts I practiced taking down the one I had, completely stripping the lower and making sure I could do that repeatedly. I still get a little surprised for a second when I cant do the burst check though, and then I remember lol...
Function check:
http://www.youtube.com/watch?v=KBHVMy2VuNo
Good video for those curious about how the AR trigger group works too:
http://www.youtube.com/watch?v=sJegVLr1rEU
Sloppy, but so easy a girl can do it!
http://www.youtube.com/watch?v=irykjLjuKo8