Being old fashioned I normally type “All best wishes, Grant” at the end of each e-mail. This is the sort of thing you ought to be able to include in your signature file so that you can save hours.
However by default Thunderbird puts two dashes above your signature so that mail clients can recognise this as your signature and format the mail appropriately. This then looks a bit dumb if you have the message sign off in a different colour to the rest of the message. To change this you can set:
mail.identity.default.suppress_signature_separator to false as outlined on MozillaZine
Then you can include the two dashes into your signature file saving me typing four words per e-mail every day. Since the 15th of June 2009 I have sent around 6403 e-mails (crikey!) so that should save me around 7631 words per year…..