We had great success using this process I learned here a few years ago:
First I made a list of the features/amenities I really wanted (number of king beds, heated pool, etc.)
Then I found a number of pool homes that fit the bill.
I sent each of them a nice note saying they were one of the homes we were considering for our vacation on (date) and would there be a better rate available.
I had emailed about 20 home owners (I wanted to deal direct, not through a management firm) and I found about a third wrote back and said their price was firm, about a third offered a small discount, and a third offered a significant discount. I only go to WDW during really slow periods, thus the big discount. One of the best deals even called me to introduce himself and to answer any questions we had about renting his home. With that kind of service we took advantage of his great offer and have stayed there four times.
With this economy I've noticed prices have dropped and negotiations are more often accepted, so I say give it a try, but be nice about it.
I'll be happy to share which property we chose. We got a 6 bedroom home for less than I expected to pay for a 4 bedroom. We've invited family and friends and had wonderful times and great service. We can't wait for our next trip in January.
Hope this works for you. Other comments/experiences welcome.