Tweaking your Signature

This is how to change the placement of your signature on Gmial, so it won't be after the responding message when you reply.
Go to your Gmail settings to "Labs." Choose Signature tweaks by Keith C and Dave C and this will place "your signature before the quoted text in a reply."

Signature / Signing Options - Email Response

Currently, there is no way with Gmail for you to make it so your signature goes directly under your message when you respond. The signature automatically gets dropped below the message you are replaying to.

For Gmail users, they will usually see your signature after your message because Gmail automatically excludes the text which is the responding message. But for people using other email serves, they will not see the Gmail signature unless they scroll down to the bottom.