Haha, I forget that all the time as well. Just send a message after it with
"Sorry I forgot..." as subject. I mean really, who cares? It is even eassier
for the receiver as he/she can now eassily delete the e-mail with the
attachment and doesn't have to stip the attachment from the original message
to clean up his/her mailbox Stupidity becomes functionality, now that's
just funny :-D
There are a couple of tools listed on the following page that will scan
your composition window for attachment keywords and prompt you to attach
something if you haven't.