I just talked about falling off the workout wagon a few days ago.
Man, is it hard to get back on, but here's what I've found to be the most effective. Just do it! I know, easier said than done right?
Here's the second part to that, make a big deal about getting back on the wagon. Make the first workout back different than what you usually do. Hit some trails, jump on your bike, go to the track for an awesome sprinting and body-weight workout. Anything that's a little different than what you typically do. It'll give you an energized feeling, and tomorrow you'll see how great you feel, and you'll be headed for your second workout without any thought.
Think about this one too, of course it's easier to stay on the couch or in front of the computer, but if health & fitness were easy, everyone would be doing it, this gives you something to brag a bit about.
Have fun, and let me know what fun workouts you have in store.