Some mail programs enable you to include the old message in your reply, so that you can reply to it point by point. If your mail program can do this, delete all parts of the old message which are irrelevant. When asking a question in a discussion list, request that people reply direct to you, and then build a summary of the replies for later publication in the list. Be careful with humour and sarcasm - it doesn't always read the way you intended it. Don't overdo signatures.
Some people add their own personalised signature to each of their mail messages.
This can soon grate on a mailing list reader, particularly if the signature is longer than the contribution. If your signature is not an option, try to cut it to two lines. Discussion lists can be fun and useful.