You may want to look at this module we developed. You can add the upselling items to your cart directly from the *product* page with this module:
http://www.bcsengineering.com/store/customer/product.php?productid=152&cat=0&page=1
Also, we personally use ( and sell) and like the ezUpsell module. If you add this product to your cart on our site you will see the ezUpsell in action (
http://www.bcsengineering.com/store/customer/product.php?productid=76&cat=0&page=) We feel the ezUpsell is a great way to clearly show to the customer that you have added a particular product to your cart and gives options of *more* stuff to buy as well as provides a clear path to checkout if desired.
Hope that helps!
Carrie