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 could follow-up every few months.

You could upsell to your customers. When they're at your order page, tell them about a few extra related products you have for sale. They could just add it to their original order.

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.

When you sell a product, give your customers the option of joining an affiliate program so they can make commissions selling your product. This will multiply the sale you just made.

Sell the reprint/reproduction rights to your products. You could include an ad on or with the product for other products you sell. You could make sales for the reproduction rights and sales on the back end product.

You could cross promote your product with other businesses' products in a package deal. You can include an ad or flyer for other products you sell and have other businesses selling for you.

When you ship out or deliver your product, include a coupon for other related products you sell in the package. This will attract them to buy more products from you.

Send your customers a catalog of add-on products for the original product they purchased. This could be upgrades, special services, attachments, etc. If they enjoy your product they will buy the extra add-ons.

Sell gift certificates for your products. You'll make sales from the purchase of the gift certificate, when the recipient cashes it in. They could also buy other items from your web site.

Send your customers free products with their product package. The freebies should have your ad printed on them. It could be bumper stickers, ball caps, t-shirts etc. This will allow other people to see your ad and order.