I recently converted an old wet dry shop vac to a "fish only" vac so I can clean out my sump better. I had a lot of trouble maintaining a siphon to suck out the detritus down in the low laying sump. Before that I maybe cleaned it out every 6-9 months. My new plan ( Thanks to Snakes thread "getting my tank on track for SPS corals") is to clean the sump for my next few water changes. I do a 10% weekly WC and it is just not enough to get everything sucked out of the sump at one time. So I will blow off the rocks in the DT, let the overflow get all the detritus into the sump, than shop vac the sump out the remove the old water for the water change.
This does two things... Remove the detritus, and harvesting the macro algae. Both of these things should help keep down nitrates and phosphates.
Having a good mechanical filtration should also help keep the sump clear of detritus (filter sock and such).
HTH's