I just wrote something relating to this. I have different things to motivate myself. Sometimes I do self talk like yelling "Let's go", which helps me take my mind off the difficulty of a workout I'm doing (sounds crazy, but it works).
I think the important thing sometimes is to just do something. It's easy to do nothing when you don't feel like working out, but sometimes a few push-ups, jumping jacks, etc. are just what you need to get going. I think not having time is a poor excuse, I'm working my ankles while typing this.
My motto is "doing a little here and there is better than nothing".