I don't think Home Depot, Lowes, allow much in the way of actual
bargaining (i.e., talking salesperson into giving you a discount from
price marked). However, I have had luck with the previously mentioned
kinds of things (damaged, floor models, etc.) and totally agree that
checking frequently is key -- being in the right place at the right
time. Recently I bought an electric range at about 1/3 off at HD
because it was an odd model they don't ordinarily carry at that store
and was missing the instruction book. As far as true bargaining, you
may find you do better at places such as ABC Warehouse where
salespeople have some latitude to dicker and deal.
Also, keep an eye out for offers such as 10% off first purchase with
new credit and so forth. The HD credit card has outrageous interest,
so make sure any discount offer also includes something like "6 months
same as cash" or you have the money to pay it off right away.