I understand exactly what you mean, but I am not sure this is possible in Entourage. I remember the same frustration with some versions of Outlook. What you want is essentially equivalent to putting the reply text after your text, instead of before -- what Entourage actually does is think of your reply as coming at the end, but then there's an option to put the insertion caret back at the top. So you get the signature at the end. I know how to fix this in Mail, but not in Entourage. 🙁