When you put your site together and build your database, put the time in to associate products with one another that reasonably go together to increase the total value of each purchase. For example, suppose your online store sells home furnishings and accessories. If a visitor puts a lamp in his shopping cart, it stands to reason he'll need light bulbs as well, or maybe a small table to put it on—so it behooves you to display links to these items for that visitor.
However you decide to associate your products, when someone puts an item into his or her cart, that's your best chance to get that person to add more items. Make sure you upsell as best as you can. How difficult this is to do will ultimately come down to how many products you offer and which inventory-management system you use. The fact is, making product associations will probably be time consuming. If done right, though, the effort should prove more than worthwhile.