We have three girls, ages 7, 5 and 20 mos.We have 2 large plastic toys indoors for the winter in the living room (cozy coupe and kangaroo climber). Otherwise, we try not to store toys in the living room, but a few wander down and stay there each day. The older girls have their art supplies in the kitchen in some plastic boxes with locking lids (to keep the toddler out of the markers/pencils).
Upstairs in the older girls' bedroom we have just their beds, toys and books. (Their clothes are in the next currently unused bedroom. Keeps the messes separate at least!) The books are in shelves that my husband built into the wall just last month. Very nice! We have about fourteen shelf feet of children's books. The upper shelves of the bookshelves are used for toys in clear buckets, sorted by type. Also the clothing closet in their bedroom is just toys. Dollhouse on the floor of the closet, clear buckets again on shelves out of their reach. We find we do best if we keep the toys up on shelves and the girls have to ask for each one, putting away one box before getting another down.
Our bucket categories: Barbies, Barbie furniture, Polly Pockets/My Little Pony, Dollhouse Furniture, Playmobil, Lego, Baby Doll Clothing, etc. Sounds like a lot of stuff, and it is a lot of little pieces, but each bucket is not that large, about 10x10x18.
The toddler has her own room currently, with just a few toys that are kept in a toy wheelbarrow as well as three small open buckets on the floor with board books lined up in them.
We get rid of stuff frequently either to charity or to friends. I agree, being honest about what your kids really play with and getting rid of the rest is the first step. Next, what do you want your kids to play with? We have just one musical instrument in the house, despite the fact that we love music, I just couldn't stand the chaotic noise of tambourines and harmonicas. On the other hand, I have a small box of wooden Waldorf figures (expensive!) that the kids rarely play with but I love dearly. I get them down occasionally but wouldn't get rid of them for anything.