If you go to a big time online retailer you will see many places on their sites that try to get you to provide an email address.  They might have a raffle or just want to notify you of upcoming deals.  Once they get the address they will typically send you monthly newsletters that keep them top of mind and while you can unsubscribe, you may well just delete them until one arrives on just the right day to drive you to buy.

This is a tried and true model and really rounds off the whole online retailer best practices.  The problem is that it is hard to get started and is a whole project in itself to get these messages created and sent.  If this is stopping you then check out EzineDirectory.com.  They offer a free membership if you send out less than 250 emails per month and it is a great way to get started doing the thing that can actually make your whole ecommerce efforts actaully pay off!