The RNT CD's suggest using these words for the quack: Hut, Hoot, Ooot, Quick, Quit, Doit, and Dwit.
For the feeder: Too-Ka, Tic-ka, Dig-ga, Doo-ga, Kit-ty, and Get-ty
For the Cluck: Cook, Shook
Be sure that you are holding the call correctly, and placing your lips on it right as well.
I just got these cd's this week, and my calls are already starting to sound convincing. I use an RNT original call. Practice, practice, practice.