I suggest keeping it simple and relatively short, focusing on the top 3 areas that are most important to you (e.g., soundstage, detail, bass presence). Then maybe have a 4th category for a few misc notes. Don't write a novel. That way you can easily glance back some months from now and have a relatively easy to read apples to apples comparison (to the extent that is even possible to do under meet conditions). I also suggest frequent "palate cleansers." In other words, take breaks. Talk to people between listening sessions. Drink water. Drink beer. This stuff easily can all bleed together. It helps to reboot frequently. Plus, CanJam is more fun--and educational--if you treat it as a social event.
Good luck and have fun. And please share your impressions. Others will benefit greatly.