Illustration Challenges circulating on Tumblr are great opportunities for practice.
I was between freelance projects at the time, so taking on this 30 day challenge was an ideal way to keep my skills sharp. Prescribed prompts made it easier to take each piece from a context-less practice exercise, to a fully formed illustration.
An specific, focused illustration every day is a good practice in discipline.
I wasn't able to finish all 30 days, but in completing the 27 I made it through, I definitely picked up some new techniques. It was also fun to be able to draw things I like, but wouldn't normally think to draw.