When I was doing navs I used to (and sometimes still do) get into the cruise and then get 'fat, dumb and happy.' Just flying along nicely with the brain resting after the busy period to TOC.
Don't let yourself do it - keep yourself busy. I get myself into a work cycle. I do a CLEAROF check, then I ask myself where I am, then I ask myself where I would go in an emergency, then CLEAROF, where I am, where would I go, etc.
Tiring at first, but soon becomes second nature.
