Check things at BabyCatalog and make sure you can use their membership discount before signing up for it (it costs $25 for one year, but you get 10% off). Some things that they sale at "special" prices don't give you the extra 10% membership discount.
Usually you can only use one promotional code per order at most online places, unless it says the code can be "stacked." You can sometimes stack codes at Amazon/TRU.
It may take a little homework, but sometimes placing two orders or ordering things from different places will get you a better deal. Some places also give you a discount if you place things on a registry and then order from there.
Also check out ordering through places like eBates. They work a little like a Discover Card in that you can get a % back added to your account and then they send you a check every 3 months (if you have over $5 accumulated). Right now you can get 7% back from BabyCenter if you sign up and click through eBates site to get there. (If you sign up, you can say that
[email protected] referred you. :-) ) I've already gotten an $18 check from them and have another $22 saved up. I am not affiliated with them, I'm just a happy customer. :-)