There are two different ways one can get discount in a 5 star hotels. 1) Business way and 2) Service issue.

1.First know the published rates of the hotel.
2.Give your company name and designation. Never know, your company might be eligible for discount in that chain or your designation might tempt them to give you a discount for the sake of business.
3.Ask in your friends and contacts, whether enjoy any special discounts, so that they can make your reservation, and get discount.
4.And, you can always talk of a long term relationship with the hotel, in case your company doesnt exist on their list of clients.
5.Usually, even if you are a individual staying in a hotel, you can get discount, in case you are staying for longer periods or taking greater number of rooms.
6.If these things are not working, make sure you go to the same hotel for more than couple of times, give some good comments, and when you go for the third time, you might be in for a surprise.
7.Another way to get discounts is to make reservation from another hotel of the same chain.(Am not sure of the U.S hotels, but try this in any Indian chains). What I mean is you go to a hotel in Mumbai, and ask for reservation for Hyderabad, 9 out of 10 times you get discount. I can guarantee of a discount, if you are already staying in Mumbai hotel.

The second way of getting discounts is not recommended, but am giving some tips, which I found out during my working with Holiday Inn.
1.Make an issue in public out of any little improper service, and any 5 star hotel seeks apology and gives you a discount.
2.Create some, if you can. I have seen guests creating issues, just to get that 5% more discount.
But, beware, these things can put the hotel staff off and might not get a room the next time.(Happens)