The previous post discussed Andrew Loomis's value lessons from his amazing book "Creative Illustration." Loomis's books, however, can be a little daunting, especially to the beginner.
In a continuing search for accessible ways to learn values, I stumbled upon the site Learning to See. Created by Paul, his goal is to help others improve their skills in the same effective way that he improved his skills. In his "About" section, he talks about being in art school in the 1980's and not learning how to draw and paint. Instead, his education was focused on conceptual and abstract art. This reminded me of being in college in the late 80's-early 90's where there was a discussion about whether or not art history majors should take studio classes. In Paul's case, since so little of his time was spent on learning the fundamentals, he was thrown out of the school!
I appreciate people like Paul who have acquired or improved a skill and now they want to dedicate their time to helping others improve theirs. He's an autodidact (like you and me!) and his before and after work clearly proves that he's in a position to instruct.
Sign up for the free value exercises and click around the rest of the site. You'll find something you can apply to your own work.