Con Law is about the only thing in law school that I actually understand. Perhaps I should be pursuing my PhD in Political Science right now instead of stuggling through this infernal nonsense.
When I can't pay attention in class I usually take a deep breath, close my eyes, hold my breath for about 10 seconds, and exhale. It does work. Other than that, there is no way to force yourself to pay attention - it's got to come from inside. You need to somehow learn to be interested in the material or trick yourself into thinking you are.