Use collect off in objects that photons do not hit. Just
put photons { collect off } in the object’s definition.
Use collect off in glass objects.
Use autostop unless it causes problems.
A big tip is to make sure that all of the final densities of photons are
of the same general magnitude. You do not want spots with really high
density photons and another area with really low density photons. You will
always have some variation (which is a good thing), but having really big
differences in photon density is what causes some scenes to take many hours
to render.