The idea would be to point the powerhead toward the substrate so that it keeps the debris moving - ideally - toward a filter input. It may take a couple to even mildly achieve this and it usually only works to a degree. And it works best with sand substrates since the waste stays on top of sand for the most part, where as it gets trapped in the crevices created by larger gravel.
Another option is to let the fish do some of the work for you. For example, I have a Geophagus Jurupari "Eartheater" in one of my tanks than continually picks up gravel and gets what he wants out of it and spits it back out. Of course, all this is doing in the long run is creating more fish poo, but it does help in between cleanings.
Implementing an undergravel filter in with your existing filtration would also help. And since you already are using powerheads, all you'd be adding would be the filter plate and uplift tubes. They really do work well as a second type of filtration, despite popular rumor.