on the days i really can't be bothered, i just try to think the sooner i start the sooner i'll be finished so just get on with it. as i know from experiance that if you skip a day or two days your normal routine becomes alot harder to do.
set yourself out a simple easy base routine that wont take a massive amount of drive to do, as the main thought behind not doing a working out is usually not being bothered as theres so much to do, so on day your feeling like that you can just do your base workout and know that your still sustaining your shape and not wasting all the effert you've put in before.