I'm no expert but my boys have shown in a local show before. We bought a conditioner feed which was mixed in with their regular feed for a couple months prior to show. I don't remember the name, so I'm probably not much help, but we bought it from TSC.
I have never fed my birds anything special before shows, but they always had a constant supply of table scraps, and I have gotten many a grand champion and best of show. The most important thing is to have good healthy birds that meet the standard of perfection and are "groomed" well.