Do you ever stop to think what your motivation is as a PC in a game world? I mean, most player advice tells you to make a character who wants something, but how often do you stop to think about it? Some players do this naturally. It is an absolute treat to behold as they just drop into character and go off playing it like a real person that no one has any control over. The rest of us...well, we're mere mortals by comparison.
Still, this simple question can do a lot for you to stay in character. How does it do that? It does it because a motivation is more than a thing that is wanted, it is a goal. Anyone can want something. A motivation is something that gives you reason to get out of bed. It drives you towards your goals and the things you want. It is the thing you want, but also the reason you want it, and the things you intend to do with it, and how much all of that is worth to you on the whole.
If you can remember to ask yourself what your character's motivation is, in answering the question you'll remind yourself about who your character is, what they are doing, and why. It gives you a means to filter through all the information you have, and look at it from the perspective of your character. Not a top down, bird's eye view that can see everything. A view from in the tunnels where the walls are real obstacles and you might not be able to see the threat on the otherside.
But why do this? Because it makes the decisions that come from you those of your character and not the player him or herself. This in turn makes those decisions more fallible. It isn't about winning or losing. It is about what is right for that character. It makes the story more personal. That, in turn, makes the game more engaging, which makes it more fun for everyone.
So next time you face a big decision, ask yourself: what's your motivation?