I would recommend closing the black tank valve for a few minutes with the black tank flush on. Often times, I will stay in the bathroom and watch the level in the black tank (our toilet goes straight into the blank tank, no elbows). I have found that just letting the water into the tank doesn't flush it out as well as closing the valve for a few minutes. The pull of the large amount of water leaving the tank helps to pull out solids that have been left behind.
As was mentioned by a previous poster, the clear extension is very helpful. When we first got one, I was amazed at how much stuff was being left behind even after closing the valve and filling the tank again. I have found that 3 fill ups after the initial dump gets most of the waste out if the tank was completely full before dumping.
However, as jnharley pointed out, if you are the type that gets easily distracted, then completely disregard my post :)! I can hear the water spraying into the tank. If I am not in the bathroom watching, if I can no longer hear it, I know I have about a minute before I have to pull the valve. Fortunately in 7 years, I have never had a problem.
-Michael