I, personally, do not invest in motivation. Motivation is a mood, and I believe that as Dan Millman says, moods are the weather of the spirit. In other words you could feel motivated right this second, not motivated 5 minutes from now. Moods are perfectly natural, so NOT feeling motivated is also perfectly natural.
I prefer to foster discipline. But most people don't understand how discipline actually works. Almost every kid I've ever met describes "discipline" with a negative connotation, as in, not getting to do the things you actually WANT to do. Or having to do the things you don't really want to do right now. But that's not what discipline is, discipline is nothing more than realizing that you don't need to feel good about a thing to do it. You can do the thing, and then feel good after because you did it. But requiring "feeling good" to do the thing assures that you're only going to do a small percentage of shit you're actually capable of. Add discipline, and that percentage quadruples.