When you make your first sale, follow-up with the customer. You could follow-up with a "thank you" email and include an advertisement for other products you sell. You should follow-up every few months.

Offer rebates on selected items. Tell your customers if they refer four customers to your web site, they will receive a full rebate of their purchase price. This will turn one sale into three sales. This cannot be done on all of your items of course, but if you are selling an eBook, or software it can double your sales.

Don