I believe in the Golden Rule (the "do unto others..." version, as opposed to the one that says "he who has the most gold makes the rules").
I believe it is better to give than to receive (at least most of the time).
I believe that if you don't have something nice to say, you shouldn't say anything (unless you are Conan O'Brien or Ricky Gervais).
I believe you should write a thank-you note (or at least a thank-you email) when someone gives you a gift or does something especially nice for you.
I believe the words I'm sorry should actually mean something, and not be followed by a preposition or accompanied by a press release.
I believe that people should get married first, then have children.
I believe that children should be seen and not heard when in public places that cater mainly to adults, like restaurants and shops.
I believe "low-fat brownie" is an oxymoron (and just moronic).
I believe a woman's bikini should have a bottom and a top. [H/T Larissa]