Capture, capture, capture.  Your website should have (in several areas) a method of capturing visitors email.  You can use a newsletter, a white paper, or other incentive but if you don't have a mechanism, to collect opt in email addresses, you are leaving money on the table.  You will always get more visitors than sales, and if you are paying for search terms the least you can get is an email address.  Additionally, every opportunity you have from order forms to in store sign ups, should be utilized to augment your opt in email lists.