For the last few holiday seasons I've worked at a book store. One of the things we do every year is a book drive for area children. Our goal is massive, around 400 books. In order to meet this goal, the store has a simple, but effective strategy. Ask EVERYONE. Even if they're frowning, even if they only bought a .50 newspaper, even if they're in a rush, even if they've said "no" to everything else you've asked them.
As a somewhat why person, this took some getting used to, but once I did, I noticed something fascinating. You never know who's going to say yes. Mr. Crankypants might just stop in the middle of his rant about rising costs, blink at you a few times and say "Sure." The sweet grandmother with the kind eyes might scowl at you and act offended you had the nerve to ask.
This is something I keep in mind when developing my characters, and writing my scenes. People have layers. There are all sorts of tiny things that affect them from moment to moment. What they show on their face or their body language might not be a true indicator of who they are or what they're thinking. They might be frowning because they can't remember a lyric to their favorite song, with no idea that they look angry. They might've just found out their kid got arrested while shopping for his expensive present.
So I try to make sure I have little moments like this in my book. Scenes where a character is thinking one thing but showing another, scenes where a character is completely misjudged or misinterpreted, because that's real, and I think it's always interesting to see the difference between the way a person is perceived, and the way they perceive themselves.
What about you, do you have any little tricks or reminders like this when you write?