If you read mail with
Rmail, set the variable mail-yank-prefix. For Gnus,
set message-yank-prefix. For VM, set
vm-included-text-prefix. For mh-e, set
mh-ins-buf-prefix.
For fancier control of citations, use Supercite (see the Supercite Manual).
To prevent Emacs from including various headers of the
replied-to message, set the value of
mail-yank-ignored-headers to an appropriate
regexp.