I think that at least for compulsives, such as myself, that one tenses up over fear of not doing something perfectly. And as a project goes on there is more to lose by making a mistake, like the moment you are standing there with a bottle of dye and a rag hoping that the color is going to be the way you envisioned it.
Cleaning the bench and sharpening the tools are good, but what really relaxes me is making a "prototype" of something. For instance, you want a new knife, instead of picking up a piece of steel, pick up a piece of scrap maple. You can work quick and freely with no care about it coming out perfect, there is plenty more scrap to play with.