Recently I stumbled across an excellent site with video tutorials ctrl+Paint. The majority of stuff discussed is something I already know (by reading the same literature the author has, presumably) but there are some things he explains clearer than I have seen anywhere else. And he also mentioned the Draw 100 approach to things we suck at. Like the author of the video, my weakest spot are arms. I'm blaming having very thin arms with no muscles for my difficulty with understanding how arm muscless are supposed to work :P and when I look at muscles in the photos, I'm just puzzled, like I can't accept that they really look that way. They look "wrong" to me. Weird, I know.
Well, here's a batch of 10 arm sets referenced from photos, using a red mechanical pencil and a 2B graphite pencil.
I have tried the "Draw 50 heads in a month" challenge before and failed. It lasted for a couple of days and I managed to make maybe 5 before forgetting about it completely. Now, I won't commit to doing this in a month, but I hope to get to 100 eventually. I probably won't bother you with all of them.