How to Format a Forwarded Message
The subject of a forwarded message can be the original message's subject, preceded by "Fwd: ", but it can also be something (completely) different.
A forwarded message should probably begin with your introduction to whatever comes forwarded. You can also add your comments to the forwarded here (although these can also go elsewhere, we'll come back to later).
Next comes a delimiter marking exactly where the forwarded message begins, for example: ----- begin forwarded message -----.
Right after this marker is the place for vital information about the forwarded message (that is not contained in the message itself): the sender, the subject and maybe the date of the forwarded message.
The recipient or recipients are not vital information, unless of course you make a point with adding it. All the other header lines are not (usually) interesting either.
Then comes the text of the forwarded message, as (hopefully) already inserted by your email program. Unfortunately, most email programs will precede it with some sort of quoting ("> "). This is not necessary (I think), but it won't hurt much either, unless you let it pile up. More about that later.
If the text is quoted, this has an advantage also, however. Here would be another place to comment it, line by line. This would work just the way it dose when sending a reply.
A delimiter similar to the one at the beginning of the forwarded text should also be present after the forwarded message to indicate where the forwarded text ends. It could look like so: ----- end forwarded message -----.