If you don't have Amazon Prime you should check it out, not only do you get free delivery, but you get music, movies and books. There is a Visa card and a store card where the amount of rewards differ based on a few things, but if you have Prime and the Visa or Store card you can get 5% credit on your amazon.com purchases.
How do you optimize your saving even further? Simple, save your rewards for purchases greater than $149. From the website "On orders less than $149 on Amazon.com, automatically earn 5% back every day as a statement credit" Since you cannot earn 5% back on order greater than $149 it will end up saving you more money.
Let's say you have $50 in rewards and you plan on buying items that cost $20, $20 and $200.
If you were to use the rewards for the $20 items you won't receive 5% back.
50 - 20 - 20 - 200 = You end up spending $190
Now if you were to spend the rewards on the $200 item first you'll then receive 5% back on the smaller purchases.
50 - 200 - (20 * 0.95) - (20 * 0.95) = $188
A savings of $2 !!! It's not a lot, but it's simple savings, just by prioritizing large purchases with rewards.