This question reminds me of the adage that the only way to never fall off a horse is to not ride. If you work out, sooner or later you're going to want to skip because you just don't feel like doing it. If it happens too often, your fitness plans (or dreams) are doomed. And let's face it, that's pretty common, and is partly why every year the gym crowds of January dwindle back to "normal" by March.
But it doesn't have to be that way. While never missing a workout would be ideal, there's a compromise: on the days you feel like skipping, feel like doing nothing, just do something. Anything.
Don't feel like doing your three-mile run? Run a mile. Make it an easy mile. Just get out there (or on there) and do something.
Don't feel like doing your 5x5 weightlifting workout? Make it 3x5. Heck, make it 1x5. With half your usual weight, if need be. Just freakin' do something.
These workouts won't get you fit, they won't force your body to adapt to being stronger or faster or leaner. But they will keep you in the habit of taking action, and that's extremely important.
You might very well find that once you get started, you wind up doing more than you expected. Maybe the dread of a brutal workout is keeping you from getting going at all. So take away the dread by lowering expectations. And if you only wind up doing the minimum it took to get you going, hey, it's still something.
So on those (hopefully rare) occasions when you really, really don't feel like working out, fine. Don't do your workout. But do something. Anything. Because doing something is way better than nothing.
Be seeing you.