One of my pet hates is people who don't send a signature with their email. A basic signature is not hard to do and makes it easier for recipients. I'm going to explain how I setup my signature in Apple Mail which is a bit of work but the result is worth it.
Firstly, open up Mail and goto Mail -> Preferences -> Signatures. Hit the plus sign and create a signature. Apply the signature to your email account by the choose signature drop down. If your happy with the result than skip the rest.
Why am I always sending attachments?
Here is the catch.. if you insert an image in the signature editor such as a logo Mail will send the image as an attachment for every email you send, including reply's. Sure, its only a small file but if you reply to a reply's reply.. you get the point, there can be a lot of wasted bandwidth.
What I like to do is upload the signature image (remember to compress it for web) onto a web server and link to it within Mail. Easy enough.. well not really. Mail makes it hard to link to the file (Its just as hard in Outlook). Your going to have to get your hands dirty writing some HTML code. Open up textedit (in plain text mode) or something similar and code up your signature. Link the img tag to your remote location. Preview in Safari as you go. Here is an example of my Launch signature:
Example HTML Signature
<div style="font-family:arial, helvetica;font-size:12px;color:#000"> <br/>Kind Regards,<br/> <br/> <span style="font-size:14px">Marc Castles</span><br/> <table border="0" cellpadding="0" cellspacing="0" style="margin-top:10px"><tr><td> <img src="http://launchinteractive.com.au/launch-interactive-signature.png" alt="Launch Interactive" width="200" height="110" /><br/> </td><td style="font-family:arial, helvetica;font-size:12px;color:#666"><div style="border-left:1px dashed #CCC;padding-left:20px">0421636775<br/> <a style="color:#666" href="http://launchinteractive.com.au">launchinteractive.com.au</a><br/> <address style="padding:0px;color:#666;font-style:normal;margin:0px">PO Box 2272, Bakery Hill Mail Centre, Ballarat Victoria 3354</address> <a style="color:#666" href="http://twitter.com/launchinteract">@launchinteract</a><br/> <a style="color:#666" href="http://www.facebook.com/launchinteractive">www.facebook.com/launchinteractive</a></div> </td></tr></table> </div>
Notice all the inline css and table junk. This is because email clients are stuck in the dark ages and support is terrible... if any. A good reference for email support can be found here
Once you are happy with your HTML preview the signature in Safari and goto Save As and save it as a Web Archive. Navigate to ~/Library/Mail/Signatures (/Users/[Your Account Name]/Library/Mail/Signatures) and replace the Web Archive in that folder (this will be the signature you created earlier). If you need to preview the signature open the Web Archive in Safari.
Close and Open Mail and you should have your new signature at your disposal. Here is my finished result: