When you call the hotel, do it during regular business hours.
Ask for the IN-HOUSE group coordinator.
If you ask anything about reservations, Hilton properties tend to transfer you back to central reservations. Central reservations can't give you ANYTHING for discounts.
Alternately, talk to the GENERAL manager. Not just the manager on duty.
Specifically have some comparable hotel rates (that are lower) in the area of the same class and caliber hotel and tell them you'd RATHER stay at the Hilton, but they need to match or undercut the other rate. Use the Hotwire / Expedia rates, they're usually the lowest for comparison.
I'm Hilton Diamond, stay almost exclusively in Hilton properties out here on the road, and this is how I go about cutting the rates so we almost always get to stay at the Hilton Property.