Going into a store that caters to folks differing from you (for my brown self, a yuppie or asian store for example) where everyone takes note of you and just act really nice to everyone. Not in a contrived patronizing way, just genuine niceness (even if it is just a few smiles), it usually sets off a chain reaction of niceness. I guess this has more effect because I look kind of choloish, but it works at all levels.

Check out any minority restaraunt in a bad part of town, and tip well.

As Chavo mentioned, drive with some courtesy (although plenty of arseholes will take advantage of it and cut you off), usually listening to jazz, oldies or classical music helps with this.

-In line, if you’re behind a difficult customer, just smile when its your turn and try to move things along as fast as possible. There’s no use in complaining about how slow service is.
-Don’t use your cell phone when you get to a cashier. Seriously, from a former barista, its just annoying.