Stay up to date, relevant and current
Make sure that the site stays up to date, relevant and current. If your site changes regularly and always has something new to offer, you are more likely to get repeat visitors, and it is repeat visitors who are more likely to spend their money with you. Make sure what you have is accessible and available to them - give something away to your visitors, even if it's just things you couldn't sell anyway, like hints, tips and pieces of relevant advice.
Freebies make people come back.
One fact that remains the same - in regular commerce as in eCommerce: a repeat visitor is more likely to be a buyer than a first timer.
A little known fact is that recent statistics have, without fail, shown that in e-commerce and online sales over broadband, the chances of a visitor buying something increase dramatically after their third visit. If someone has been to your site three times, there is something there that is of great interest to them, and, consequently, it is more likely to be something there that they'll consider buying. (The same is true of sites that offer services, or act as brochures and hooks into real world trading rather than just e-commerce. A repeat visitor is more likely to pick up the phone and call you than a first timer.)