Including the relevant section from the original message helps the recipient to place your reply in context. Include only the minimum you need from the original message. Quote back only the smallest amount you need to make your context clear. This makes the reply much easier to follow. Many mailers can add a signature to your messages automatically.
Signatures are a great idea but are subject to abuse.
Keep your signature short - four to seven lines is a handy rule of thumb for maximum signature length. The length of your quote adds to the length of your signature. There are hundreds of others, some more recognizable than others. Like any embellishment, however, overuse of smiley faces destroys their value - use them sparingly. The Bottom Line Above all else, remember that electronic mail is about communication with other people.