Things I do now that I didn't do then...
1. Buy an AP for one member of your party and go when you can get an AP rate for the room. I, too, paid rack rates once- never again...
2. My favorite stays have been during the first two weeks in December. If you can do this I highly recommend it.
3. Stay onsite no matter what, and use EMH for at least the MK and AK.
4. Take a 3-4 hour break in the middle of every day (except AK visit). If you get up early, you will have all the major rides done by lunchtime- so taking a swim and sleeping a couple hours before going to a park for the evening is easy to work in.
5. Don't be 'militant' about seeing and doing everything. WDW isn't going anywhere- what you miss this time you can see the next. Make a point to visit all the major attractions, see the others if you don't have to stand in line more than 10 minutes or so.
6. Read the Unofficial Guide, get a Passporter, and subscribe to TourGuideMike.com. Obviously you are posting/reading here, which is another thing I would have done. Mousesavers.com posts a lot of codes, and the information at allears.net would have been welcome the first time around as well. Time is money at WDW- and the majority of tips and tricks you learn from these resources will save you a couple of hours of standing in line a day, as well as a ton of money on your trip. I always feel badly for the families standing at the entrance with the park map/schedule opened up and looking around in bewilderment- PREPARE.
6. Stay at a value resort, even if you can afford a deluxe. I now take the difference in rates between a value and deluxe and put it in a savings account for my next trip as a reward for being frugal. I have 1/4 of my next trip saved for before I leave for the current one. This tip will ensure you are able to come back every year or two if money is an issue. This helps to make each trip relaxed, because you know there is no need to do it all- you will be back soon

It is a resort vacation afterall...plan for a marathon, not a sprint.
I could write all day long, but those are the major things. Even if these were the only changes I made from my first stay at the Poly I would have saved around $2000.00 on room costs, rental car, airline, etc- not to mention all the time spent in lines.
Blessings,
mic_key_mouse