One thing that all of us seem to agree on is that humor is difficult partly because it’s so subjective. As Eleanor says, what’s funny for me might not be funny for you.
One thing that Sean and I talked about when we first discussed this issue is that I’ve come to believe that one thing a writer needs to convey in order to successfully pull off interpersonal humor (like sarcasm, as opposed to say, slapstick,) is information. Interpersonal humor works when we _know_ the quirks and foibles of the people involved. They need to be people we can laugh with (or at). To do this well, in my opinion, you have to be superb at characterization.
I suggest then that one of the best places to start is by establishing your main character as someone the reader can trust to not mock people out of meanness, but out of a kind of appreciation or even love. Interpersonal humor is difficult because I think that there _is_ a fine line between laughing with someone and at them. An author needs to approach that division very carefully. I agree with Kelly that one of the best ways to deal with the issue of “meanness” is to remember that it is far more sympathetic for a main character to poke fun at themselves than at others. So, if you can establish your view point character who happily laughs at their own stupidity, I think a reader is then better able to accept when s/he pokes fun at other people’s idiocy, because you know s/he’s someone who is, at the very least, willing to laugh at her/himself for the same “mistakes.”
It establishes a baseline kindness. To be successfully snarky or sarcastic in text (and perhaps in life), I believe you ultimately have to be decent and nice. I think this especially important if you chose to introduce characters who are larger than life. I’m thinking of the character grandma in the Stephanie Plum series (by Janet Evonivich, ONE FOR THE MONEY). For me, those kinds of crazy, almost unrealistically hilarious characters work because Stephanie Plum loves them. She observes their insanity through the filter of familial bonds based in love. Even her father, who could read as dark and cantankerous reads as sweet and lovable because Stephanie’s word choice and description of him make her own appreciation for him obvious to the reader.
Just my two cents. Arguments? Agreements?