A common mistake you can often find in eCommerce site designs occurs during the checkout phase and it's another example of a sort of Real World / Web World fallacy. We'll start by comparing our web store's checkout process to a retail checkout line - they share a common name afterall. If we design a checkout process using a physical store analog we might consider adding all sorts of features; displaying products that are related or recommended, seasonal, featured, etc, etc. Just like the candy and magazines, or batteries, gift cards, whatever at physical stores. This seems reasonable - we've got a customer who's ready to spend money after all, why not try and get them to buy something else while they're ready?
The problem is that the physical checkout line analogy is flawed. This is the internet, there is literally no checkout line. The reality of accepting payments and needing to ship a physical product still forces you to get information from the customer, but the analogy is closer to the actual transaction between a customer and clerk at the register itself, not waiting in any physical line. When we consider this comparison, the idea of distracting your customer - potentially compelling them to navigate away from the checkout process - can harm your overall sales.
With this in mind, one simple way to increase conversions for most eCommerce sites is to remove navigation links during checkout. A recent and very comprehensive eCommerce usability research study confirms that the top grossing eCommerce sites also remove their navigation links during checkout http://baymard.com/checkout-usability/benchmark/top-100. Other product links and advertisements potentially encourage the customer to navigate away from the checkout page; to browse and compare, potentially to leave your site and checking out others. Once a customer has decided to buy a product, take their money and be happy.
Now that doesn't mean you can't implement some of these features elsewhere. Display related products on a specific product's page, suggest extra items when they're viewing their cart before checkout, you can even continue with suggestions after they've made a purchase; either through specific suggestions after checkout, or via email follow-ups. But remember, once they are in the checkout line and ready to pay - never take them out of the line.