The idea of planning your motivation strikes me as not only a strange concept, but also a little pretentious. At the same time, this is a necessary step in making a believable, fleshed-out character. I'm sure you've all had that moment where you're reading along in a relatively decent story and you realized that something a character just did or said doesn't line up with what they previously said they wanted, and there's no explanation given at all.
The queen was generally a kind woman, and well beloved by her people. She dressed in nice clothes, gave her leftovers to the poor, and never traveled when she could avoid it. "What are you doing, Sir Garrison?" she asked brightly, coming across the gallant knight in her garden one day.
"I'm practicing my artistry, Your Grace." Sir Garrison lifted the canvas so she might see the first rough lines of his composition, recognizable as vague tree and cloud shapes.
"Well, that's silly. Everyone knows that knights can't paint. Maybe you should work on your swordplay instead."
Just writing that made me gag a little. Sorry. This is a pitfall that I struggled with for years before I finally worked out for myself that simply saying a character is kind isn't enough to make them a kind person. In the same way, saying a character really super wants to save the world, then letting them doodly-do around and chase the hero instead of actually trying to save the world isn't enough to convince the audience that what you say about them is actually true.
This can be a really powerful tool in your authorial belt, though, if it's played right. How many times have you heard a friend say "I really want to do such-and-such," and later on your friend realizes that it wasn't actually such-and-such that they wanted to do, but some subcategory of the such-and-such that appealed to them? That can happen to anyone, and when it happens to a character (Oh, I thought I wanted X but once I finally got it, I realized that it wasn't what I wanted) that can make a character seem much more real to the audience.
The door was locked. Charlie glanced cautiously around, but the hallway was still as empty as it had been. Praying this wouldn't make too much noise and attract attention, she pressed her hand to the lock and gave it a twist, as if the key were sticking out of the middle of her palm. The lock clicked open and said (very audibly) "Welcome back, Professor." Charlie winced and held very still for a moment, trying to listen over the pounding of her heart for the sound of approaching footsteps.
It's hard to give you a good example in so small a space. The problem is that, in addition to her obvious desire to be sneaky in the snippet above, Charlie's motivation for entering the professor's office secretly must also line up with the rest of the story.
So as a quick rule of thumb, here are three tips:
1. Don't state motivation where you can avoid it.
Instead of saying "she wanted to steal X" or "he wanted to date Y," just show them pursuing that goal and let the audience work it out for themselves. That eliminates the possibility of giving your character a chance to prove you wrong.
2. Play with the character before you start your story.
Assuming you have the time to do so, stick your character into 3 or 4 different situations and settings. Make them deal with different people and see how they act and what they want. (An awesome example of this might be Dustfinger from Inkheart, literally taken out of his world and put in another one, where his goals remain largely unchanged.)
3. Have your character achieve his goal - now what?
Much like Inigo from The Princess Bride, there are some characters with a goal that doesn't lead any further than the goal itself. This can happen in real life - has your character thought about what comes afterward, or is this something that they'll have to deal with later on? In achieving their goal, do they discover something about the goal or about themselves?
If you're going to put your time and effort into writing a story you care about, definitely don't forget to have fun - but don't do it half-heartedly, either. Your story deserves love from readers that aren't you, too. ;)