Fortunately, placing your signature is probably the easiest part of having it in your emails, and that's not only — not even chiefly — because there are no clear-cut rules. I think the most appropriate place for your signature is right where you'd also sign a written letter.
Where to Put Your Email Signature
Put your signature:
- right below the end of your text — in replies as well as in new messages.
- If you include original messages using indentation and selective quoting, your signature will usually be right at the bottom of the message.
- If you quote lazily by putting your message on top of the (un-fragmented) original message, your signature will usually be between your message and the original message you quote, just above the "--Original Message--" line.
Naturally, positioning your signature in the suggested places, gives you the option to include post-scripts (PS:) right below it.
Note, though, that some email programs and services treat anything below the signature delimiter specially: they deem it especially unimportant. So, as an alternative, you can include your postscriptum below "signing" your message's main text with your name, but above the email's signature.