Tolerance. Thanks to Heath's comment on Essentials I, this got added as a mental essential for bike commuting. It tastes even better if you season it with a big dash of humor.
He's absolutely right--you have to be tolerant of those people at work who look at you like you're crazy (especially on days that are on the rainy or cold side). You might point them to my post that compares the hassle factor of bike commuting compared with driving (although it makes more sense to someone who's been through the complete psychological conversion process :D).
You also have to be tolerant of drivers who tell you all their stories about bad bikers--those sidewalk-terrorizing, helmetless scofflaws (or just cyclists who take the lane as they're legally entitled to).
If you ride the Centennial Trail you'll encounter pedestrians with baby strollers who think the "wheels only" lane in Riverfront Park is for them, people who are positive(ly wrong) their unleashed dogs are perfectly polite and would never take a chunk out of a passing cyclist, toddlers who zig when their parents think they're going to zag, rude cyclists who whiz past without yelling "left!" to let you know they're sneaking up behind you, and other joys of sharing a public space. Tolerance. Tolerance. Tolerance. We're all in this together.
Persistence. You can’t try this bike thing once and then quit. After all, that’s not how you learned to ride a bike in the first place.