Many mailers can add a signature to your messages automatically. Keep your signature short - 4 lines is a handy guideline for maximum length. The length of your quote adds to the length of your signature. Avoid topical or local quotes, since they may be meaningless to recipients in other towns, countries or cultures. Remember, too, that humor is often cultural.
Variable signatures are usually best if they're cute or amusing.
Outbursts on politics or other such topics will turn most people off, but a one-liner that brings a smile can make someone's day. While this might sound trivial or even insulting, it's astonishing how many people who are perfectly polite in everyday life seem to forget their manners in their e-mail. Don't expect an immediate answer. The fact that you don't get an answer from someone in 10 minutes does not mean that he or she is ignoring you and is no cause for offense. E-mail is all about dealing with your communications when you are able to do so.