We created a shelf down in our basement where we store ALL the toys. We call it the Toy Library or Toy Shelf. About once a week we have a "Toy Swap" where we exchange the toys that they have had upstairs that week for a new set of toys for the week. Usually we only bring up 4 or 5 activities at a time.
We got this idea (and many more) from a great book called Simplicity Parenting (follow link below). It is a fantastic parent read and I would highly recommend it!