Don't expect an immediate flood of traffic, but since you are sending out emails for business anyway, or if you regularly post in a forum, then you shouldn’t let the opportunity of adding a signature file to these kinds of communications go unanswered.

A signature file is simply a link back to your website (or a specific page on your website) that you can use in closing a communication. 

Example 
Sincerely,
Laura Theisen
www.precisionroi.com

Forums will generally allow you to add a signature file providing your post or signature isn’t a blatant “sell”.  If people like you input in the forums they will want to see what else you have to say in your site – so make it easy for them!

Another effective use particularly in email communications is to make the signature link a Postscript.

Example 
Sincerely,
Laura Theisen

PS Signup to receive our newsletter (where "newsletter" is a link to your newsletter sign up page/form)

There’s never a single silver bullet for doing anything. Success generally comes from the combination of many small things working together and this is another to add to your arsenal.