Also, some put a couple of bags of ice down the loo, as this is supposed to cause a scrubbing action as you drive down the road..
A couple of tips.. After draining, make sure you put some water back in the black tank.. I put 5 gallons (mixed with a cup of liquid tide and a capful of liquid calgon.. really smells nice!).. Don't want stuff left at the bottom to dry up.. Also many newbies freak out that their tank isn't draining all the way, because the gauge shows 1/2 full .. Don't trust the gauges. They regularly get stuff stuck to them causing a false reading. I have pretty good luck with the GEO method + ice.
Also, if you have a tank sprayer built in, you can connect a hose to it, and it creates a violent (maybe rotating?) spray inside, to help get everything out.