I follow what I like to call the “Promise” principle.
Something that I have learnt over time is that it is difficult to rely on will power alone to get things done. This is because will power of an individual can be depleted easily. A day of hardwork would already have sapped a lot of your energy and there is no way that you could motivate yourself to do something again if you had not foreseen such a situation. So, if it is something that you have thought about doing only on that day, then it would be better to leave it for some other lighter day and get a good day’s rest.
However, it is a whole different matter when it comes to things you want to do regularly. What I have done is to promise myself that there are some non-negotiable things that I will do irrespective of whether a day was hard or light. This promise is a commitment that I have made to myself, and there is nothing that I would not sacrifice to see to it that they are not broken. An example for such a promise is my reading habit. I read everyday irrespective of how hard/easy the day was. I squeeze it into whichever small pockets of time I can find in a day.
As time goes, and the unbroken streak continues, my commitment to uphold the promise also becomes stronger thus making sure that I do the things that I want to do irrespective of the day’s nature. My motivation to uphold my unbroken streak gets stronger each day, thus becoming a wholly sustainable model. Over time, many of these have evolved into habits, thus coming to me as second nature.