RobJ,
You're right. The trench does not stay cleaned out at all. I think the trick is to either pull the pipe into the trench with the subsoiler, or feed it through a gentle 90 degree bend pipe attached to the back of the subsoiler. Cleaning out the trench with middlebuster might work for a shallow trench, but my subsoiler tip will dig deeper than the middlebuster will. The water line needs to be down about 12", ideally.
OkieG