(Excerpted from Only Read the Fine Print the book)
There are two types of motivation - positive and negative, or "closer to" and "further from". Either you want to get closer to some achievement or possession or status, or you want to get further from some problem or embarrassment or punishment. Regardless of the exact circumstances, however, those are the only two reasons people do anything - to get closer to something good or further from something bad.
This even applies to the things that people "need" to do. People don't actually need to go to work - what they really need is to eat, but since they want money to pay for food, they choose to go to work. They want to get closer to food and further from starvation, so they want to work to pay for food. They do it because they want to, not because they "need" to. And yes, there is an alternative - grow your own food. It's harder than most jobs and puts less food on the table, so people choose not to do it because they don't want to.
Things that people are "supposed" to do follow the same rules. People are "supposed" to obey the law, but the reason they do so is not because they understand that just laws are necessary to keep society from devolving into anarchy. They obey the law either because they want to enjoy the feeling of being a good citizen, or because they want to avoid going to jail (or both). Feeling like a good citizen is something you want to get "closer to", and going to jail is something you want to be "further from". Either way, the intellectual ideal of law and justice doesn't make you obey the law - your desire to enjoy positive consequence or avoid negative consequences does.
There are two important ways to apply this knowledge: motivating yourself and motivating others.
When you want to motivate someone else (notice I said "want" and not "need"), remember that they're only going to do something if it helps them get something they want. The best way to motivate someone is, therefore, to find out what they want or don't want, and find out how you can help them get closer to or further from that. Help them get closer to what they want, and they'll help you get closer to what you want.
When you want to motivate yourself identify what is important to you, then decide how you can get closer to or further from that. Don't worry about what you "need" to do, or what you're "supposed" to do - focus on what you want, then decide what actions you need to take to get it. If you decide you don't want it bad enough to take the actions necessary to get it, that's ok - be honest with yourself about that and move on. But if you really want it, nothing can stop you. Focus on that want - the goal - and it will drive you forward.
Images used in some versions of this post are courtesy of Stuart Miles at FreeDigitalPhotos.net