Sometimes I listen to that voice that says to take a day off, if it's because of tiredness, for example. But I use several types of motivation.
First, there is the negative motivation: if I don't keep fit and exercise, I may end up like our former department secretary: overweight, bordering on diabetes, unable to waddle more than a few yards without getting out of breath. Also, I remember back to the time, just a few months ago, when I had gotten out of the exercise routine for a while, and the mirror in my exercise area was not a pretty sight to behold.
Then, there is the positive motivation I get from feeling and seeing the progress I've made and contrasting it with that unpleasant mirror image. Also, our new department secretary is young and fit, and I'm somewhat trying to "keep up" with her.
Then, there are those days when I just don't feel like exercising, but I know it's mental exhaustion (a long day, students being at their most annoying, my starting to develop a headache) and not physical fatigue and I SHOULD exercise. This happened just a few days ago, when I was scheduled to do Boot Camp. I was about to just throw in the cards and reschedule the workout until the next day, but I decided to go through with it. My method of psyching myself up seems rather bizarre, even to me. I first set up my interlocking flooring (not bizarre) and got my weights ready). Then, I went to my workout wear drawer (actually a big Rubbermade container) and started sorting through my choices, in search of the "perfect" outfit for the day (as if it would matter. I mean, the only ones to see it are myself and my cats!). As I was sorting, I narrowed it down to two choices: blue for calming or red for energy (which I actually commented on to myself). I finally chcse red for energy. My workout went really well, and the beginning of a headache disappeared about 15 minutes in.
I think that when you do something repeatedly, there is a certain ritual associated with it (like setting up the area and choosing the perfect workout outfit) that your subconscious gets used to. As you move further into the ritual, your mind starts to get your body ready. I've read that people who run on a consistent basis get to a point where their heart rate begins to rise a bit when they put on their shoes--their mind knows what's to come and starts to get the body ready.