How you phrase a discount is a powerful way to increase conversions. Some visitors do not like to do math, or will do it wrong. Therefore, offering 20% off is less effective than save $18. However, high discounts (50%, 90%) may draw buyers more powerfully than the dollar value.
You have to find out for yourself.
Karon offers a nice list of alternatives and some links to research for your enjoyment.
Group items tagged 
You have to find out for yourself.
Karon offers a nice list of alternatives and some links to research for your enjoyment.