A friend of mine has those large plastic tubs with the lids, They seperate the toys into the 3 tubs, 2 tubs go into the shed (out of sight/out of mind) and the one tub inside that can be pulled out and played with, etc. Every couple of weeks this tub gets packed up and rotated to the shed, the kids get a new tub from the shed of toys they've forgotten about.

Because the kids forget about the toys that have been packed away, my friend can then get rid of things they normally wouldn't let her part with.