This is a fascinating topic to me. I think willingness is an underrated quality in people; people value things like potential more than willingness, because willingness is "only" the thing that bridges potential and reality. It's easy to say, "Oh, just do it," but I think everyone has to figure out the best ways of cultivating willingness in themselves.
My solution so far is only to do athletic activities I enjoy enough to do on their own even if they were not making me any healthier. Even then, I go through periods of being super-bored with bouldering, I burn myself out on cycling by training improperly and then pushing myself too hard, or I just lapse on the yoga because I don't stick to a schedule.
Lots of other good advice on here, which basically boil down to really doing some self-analysis when you're lacking motivation and figuring out why.
I always used to write off as "crutches" things like committing to future events that required me to train in the meantime, or relying on others to hold me to training, or basically anything external. But that's silly; if I have those external things, I might as well take advantage of them, and there's nothing wrong with, say, committing to a really long bike ride a few months off which overwhelms even my finely-trained procrastination instinct and gets me to train because I know if I don't, I'll be toast when I have to do that ride.
For the most hardcore example of the "eliminate the barriers" strategy I've ever seen, check out this guy's blog:http://onmytwowheels.blogspot.com/2007/01/neither-rain-nor-sleet-nor-snow-nor.html
He started systematically breaking down his reasons for not cycling... to the point where he will bike daily no matter what, even through up to 4 inches of snow and blistering cold! That's a bit more hardcore than I am, but the principle is sound -- if you are feeding yourself excuses, take advantage of that information to eliminate them one by one.
Another practice I've found useful from the Buddhist side of things is to view negative emotion and experience not as reasons to be upset and dejected, but motivators themselves. That is, when you're pissed at yourself about missing workouts, or you feel a sense of dread when you think about stepping onto a scale or going to lift after taking time off and fearing the loss of strength, oftentimes (if you're anything like me) those negative emotions just make you sit there and suffer and fight with them and make it all worse.
Instead, you can put them to use -- when you feel fear or dejection or whatever, channel it instead as motivation, i.e. "This fear is my signal to go work out, so I stop feeling fear," or "This depression is my encouragement to work out, because I know if I do it'll go away." It's a kind of mental switch, and it's hard to maintain all the time, but when you do it's pretty cool. Free motivation! Plus, redirecting them like that means you don't sit around beating yourself up, which is critical, because that does the exact opposite, lowering your self-confidence and increasing the chances you'll lapse again.
Enjoying the process itself is important to me, too. I used to get really upset when I would go boulder and get stuck on a problem and be unable to do a particular move, and get frustrated. Then I didn't want to come back and work on it, either. A friend casually observed, "Yeah, the key is to enjoy the process, and not be attached to the result." Now, I won't say I necessarily love the days when I go in and don't send a single problem, but I'm more able to work through them, because I can focus on enjoying the process of working on the moves, enjoying figuring things out step by step, and not judging my work based on whether I "succeeded" or not. After all, in exercise -- like in all things! -- there's no such thing as "success." We work and make progress, but we're never "done."
Heh, sorry this is so long, I'm putting off going to sleep. Hope it's of some use! :)