The principles that guide these tips are simple: we want our e-mail list to be interesting, civil, and manageable. Interesting means posts that are on-topic and make for good reading. Civil means posts that treat list members with respect, though it doesn't rule out a good argument once in a while. Manageable means fewer e-mails are better than more if they say the same thing. We all have enough noise in our lives.
PLEASE EDIT YOUR POST. When you reply to a message, please don't quote the entire previous message. Please take an extra few seconds to edit the amount of quoted material down to the bare minimum so that your reply will make sense. This especially helps the digest version of the list, where one long reply thread might fill a whole digest message.
CONSIDER ASKING FOR REPLIES TO GO TO YOU. If you're asking for a contractor reference, for example, you might consider asking people to e-mail you directly and offering to summarize the results. When you do this, you provide a very helpful service to the entire list, because you consolidate all the useful info into one message which can be saved if it's of interest, or skipped if it's not.
ASK YOURSELF: WHO IS YOUR AUDIENCE? Think about whether your message is of interest to the whole list, or just the person you're replying to. If it's just the person you're replying to, consider sending it just to that person rather than the whole list. Note that when you reply to a list message, by default your reply will go to the individual person; if you want it to go to the whole list, you'll need to edit the "To:" field of your reply. In any case, please note who your reply is going to and make it a conscious decision.
TAKE A DEEP BREATH. Did someone just say something that annoyed the $#@! out of you? Don't you just want to write back to the whole list and tell them what you really think of them? Well, go ahead and write it! Just don't send it for a few hours or a day. Re-read it before you send it. Rewrite it so it's civil. We'll all appreciate your efforts.
WHEN IN DOUBT, ASK. I'm happy to answer questions of style, form, etiquette, or even spelling :-).