Assuming each product as a "Description" field in your products table...
It would really depend on your database's table design. What are some of the fields in your products table? Do you have a category or description field? If so you could just show similar products based on those two product fields.
For example, I was building a small site for a company that was selling car parts. My products table had a field called PartType (or something like that). So if a user added an intake system to their cart, I'd display a suggested parts section that would include air filter clearners, mass airflow sensors, header pipes, and high flow exhaust systems.
Do you get what I'm saying? You just have to logically pair whatever the user adds to the cart with similar products that they may be interested in.
