I'm currently doing an improvement challenge specifically designed to be challenging and foster artistic growth, without taking too much time every day. That's actually how I found this site- the friend I'm doing it with suggested we use this site for day 6.
http://heartache-hit-the-trail.tumblr.com/post/125266123421/30-day-improve-your-draws
I haven't figured out how to change my icon or post links as links, but if anyone's interested in this, you should be able to paste the link into your browser.