alternately, i find that asking questions and engaging the speaker helps, but some speakers just plain suck.
"I can never be what I ought to be until you are what you ought to be. You can never be what you ought to be until I am what I ought to be. This is the interrelated structure of reality." -Martin Luther King, Jr.
If there is a textbook for the class, read the related chapter while the instructor is speaking and keep a count of how many exact phrases s/he "borrowed" from the text.
Establish a rapport with the instructor if the class is more than a single seminar. Nothing kept me awake at school like waiting for the chance to get in a clever dig on my professors.
Show up five minutes or more late. Piecing together what you missed is a good activity to keep your mind active, and it has the added bonus of making you feel even more embarrassed if your prof catches you napping. This only works so long as you retain some level of shame, though. My personal shame at being late to class always went away after I correctly answered one of my professor's questions. Then I felt free to nap again.