Jamie,
Capturing the email address of visitors to your site as well as your paying customers is the goldon rule of internet marketing. It is something that should be planned for and implemented when setting up your site as it is a key part of the marketing process. Your site should be getting 1000+ unique visitors a day if you're doing your marketing right and your ad copy will be converting 2% at least if it's up to scratch but the other 98% will **** off without doing business with you. Your goal is to capture their details weather they do business with you or not so that you are able to communicate with them in the future with the aim of converting them into a paying customer.
The trouble is too many people online play the short game, out to make the sale and the big bucks there and then with no back end or future plan, this is SO shortsighted!
The most effective way to market online and capture details is to use a loss leader, give something away to the visitor to your site in exchange for their contact details, this immediately starts the pattern of them trading with you, they know who you are, they'll remember you and this relationship building is key. The next step is to follow up a day or so later while you are still fresh in their mind with a simple email enquiring how they got on with their free product, this way they get used to your communications in their inbox. The next communication would be offering them a product but with a sizable introductory discount, obviously time sensitive and you then begin to funnell them on to larger ticket items - this is the importance of a back-end. Start with a loss leader to build your list and make your money on the back-end.
It's all about understanding customer aquisition cost and the lifetime value of your customer, once you know these figures you'll have no problem with making a loss on the first offer.
If I know the average lifetime value of a customer to me is £100 per annum, i've got no problem with giving away £20 notes as an incentive to start our business relationship in the first place!
Hope this helps.
Robert