I've added a portion to the forum rules on quoting others.
With the advent of email subscriptions, it is important to realize that email clients often append the full text of the message replied to. This can look messy on a forum (and it's largely unnecessary most of the time). So, the added portion is to tell people to be sure to remove such, or only quote what is necessary. Also, sometimes people do this without email subscription—so it's also meant to be a heads up for them.
Don't for get that you can use an ellipsis to indicate that something is left out: i.e.
> Here is some quoted material …
