Have Upsell, Frequently Bought Together, Related Items, You May Like and Recent Views recommendation boxes enabled in your cart page. This is the last page your customers will see before checking out. It is very valuable to upsell/cross-sell items to them in this page.
Upsell - above the cart items
Frequently Bought Together - below the cart items
Related Items - below Frequently Bought Together
You May Like - below Related Items
Recent Views - below You May Like