That's the key to maintaining a routine over the long term, actually enjoying what you are doing. You can't fight against your will indefinitely. You need to find exercise you enjoy doing and then motivation will not be a problem.
I try to include my workout during my day so it literally becomes a routine for me. Having a good playlist as well as going to a a good gym makes it easier. To be honest, I really enjoy working out sometimes because it relieves stress that can sometimes build up throughout the day. As a failsafe for the days that I really, really don't want to workout, I always try to make my friends hold me accountable. I talk about my goals and what I am trying to do with them which makes it no longer about letting me down but people that I'm around down. It's kind of corny but it works.
