ToysRus tends to price their lego higher than suggested retail, which allows them to offer "great" discounts.
If you do Upromise, you can get 3% back on Lego purchases from lego.com, toward the college fund. Over time, it does add up.
Depending on what you are ordering, you may want to check out:
http://www.legoeducation.us/store/
and
bricklink.com (new AND used lego sets and parts)
I have some Lego to order also, hoping for more discount ideas!