I suspect there are some secrets out there, but I do just fine with RIT and heat. I float my stencil covered disc in a warm dye bath of water + RIT. I used to apply the dye directly to the disc, but that was very inconsistant. Full contact with heat is the key... but not enough to boil!! Don't leave it in too long either because it will soak up more dye than needed and tend to bleed in a shorter period of time. Here's my general rule for soak times to get a dark black: Pro = 10-15 minutes, Star = 15-20 minutes, Champion = 30-40 minutes. Lighter tones are less of course.
I also have used "worm dye" (used to dye fishing lures) and it works very well without having to use heat... just expensive though.