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.