When not in use, trammel heads should be kept somewhere safe to keep them away from moisture that could cause rust. Their small size means that they can be easily stored and transported in a toolbox.
Cleaning and oiling your trammel heads every so often will ensure that they continue to function correctly.
In particular, you should check that the adjusting nuts and clamping nut of your trammel heads are not stiff and are working properly.
If your tool has a sharp blade attachment for cutting circles, it is likely that you will need to replace the blade after considerable use. Some trammel heads are supplied with extra blade segments with which to do this.