It's great that you are wanting your students to have this experience.
"Token Response" is an engaging way to get middle schoolers thinking about this. It was developed by Mary Erickson, Ph.D. and Eldon Katter, Ph.D. and is available thorough Crizmac http://www.crizmac.com/
One of the things I also use is a check sheet when introducing this. They become comfortable with the process and realize that they can do this on their own! For example "what kinds of lines do you see" and a list of descriptive words and they check off the ones they see, etc. I start 6th graders with criticism so by the 8th grade they are pretty confident looking at works of art.