The Force Morale System in Dux Britanniarum is a key part of the game, and is easily tracked by a D6 or two on the table edge.
However, a far smarter arrangement is to use the Force Morale Trackers designed for the rules, and great when used with a suitable figure placed on your current morale level. I like to use a priest for the Romano-British and a wicca or shaman for the Saxons.
You can download the Force Morale Tracker here (PDF format).