I came across an interesting concept recently. Muscle confusion. The idea is to expose the body to the greatest variety of movements, intervals, loads, modes of action as possible when you train. In other words, you treat your body as a complex system, not as a simpleton machine. I think the concept makes sense.
I have been doing push-ups and sit-ups these days, and what I find is that monotony leads to boredom. After a while you simply cannot take it any more, since your body knows already what to expect and it is not fun.
It is interesting to consider how you can expose your body to different kinds of physical trials, at quite unexpected times. In other words, you always try to take your body by surprise.