We use the "GEO" method (there are many links on this site) that uses ~1 cup of Calgon liquid water softener in each of the holding tanks and ~1/2 cup of liquid dish washing detergent in the black tank. But on saying that, I only do it about 1/2 of the time, the rest of the time I use nothing. I don't like the smell of many of the commercial holding tank chemicals. There are a few RV Parks with septic systems that will not let you use chemicals.
Do what "the bear II and DarthMuffin" suggest with the valves.
Some people place ice cubes in the holding tanks to rattle around while driving to help clean the tanks, which may or may not help.