Pierre Neidhardt writes:

> This is what I do to sign all my messages:
>
> (add-hook 'message-setup-hook 'mml-secure-sign-pgpmime)
>
[...]

A word of caution: at least with Gnus, doing something like this might
"downgrade" the security of a message, by changing it to only sign the
message you're composing when normally it would have been both signed
and encrypted.  It typically happens when replying to an encrypted
message.

For that reason, I set the message to be signed only when it's not being
signed+encrypted:

    (add-hook 'gnus-message-setup-hook
              (lambda ()
                (unless (mml-secure-is-encrypted-p)
                  (mml-secure-message-sign))))

For non-Gnus users, I'd imagine replacing `gnus-message-setup-hook' with
`message-setup-hook' would work.