Cross selling is a good way to show more related products when your customer purchases a product. When you are creating or editing a product, simply click the "Cross Sell" tab, then select the category from the drop down menu of the product which you would like to cross sell. All of the products in that category will appear in the box below. To link two products together, simply select the product and click the arrow that points to the other box. If you make a mistake, just select it again and click the arrow going the other way! You can repeat this process until all of the related products are listed in the right side box.
After you save your list of cross sell products, they'll show up when the product is added to the shopping cart.