you box up and hide box for a couple of months. if dc remembers something they want from box, they have it but after two months the box goes on freecycle or I take to our local childrens centre for their table top sale.
alternatively, I arrive in my dcs room with a black bag and anything not put away goes in the bag and off to children's centre. My dcs are older though and this usually makes them tidy up!
another option to reduce toys coming in is that family now contribute to one big pressie at xmas and bdays or buy things like an annual pass for somewhere or theatre tickets.
we also practise the one in, one out for any toys purchased outside of presents.