Types of cross-sell and up-sell promotions:
- You can promote best sellers that are related to the item that user is browsing right now
- You can offer discounted bundle with completentary products (like Amazon does with Frequently bought together section).
- You can promote other product collections
- You can show products based on previous purchase history.
But when adding such sections, take into consideration that it can increase AOV and Quantity per transaction, but at the same time it can significantly decrease conversion rate because in that case you will motivate shoppers to browse the store instead of streamlined purchasing flow. So don't make such sections very prominent and don't place it too high on product pages, unless you are A/B testing it.
Based on my experience, such sections works well for Desktop and Tablet users, while have negative impact on Mobile users because shopping behaviour and context is so different for these segments of users. Also it's better to place such sections somewhere below product page description or reviews sections. Example of 2 cross-sell and upsell sections on Amazon below the first screen of product page