I use a combo of:
mousesavers
flyertalk
fatwallet (has a big thread)
e-mails from the company
entertainment book (if you have it)
Codes that can give you the best discount in one location may not be the best the next trip or a different location, although AirTran was the best on the last two trips (Vegas & this one).
National has also changed their modification slightly. It doesn't appear to raise your rate if you modify your ressie & rates have increased, but it no longer decreases it if rates have dropped. I have to start a new reservation & see how it is pricing and then either confirm or just stop the reservation.
When you read through the code threads always make sure you use a code you actually are eligible to use such as Walmart, an airline program, etc. Some of them are for certain gov't agencies, etc.