Start by sorting toys into categories - games, blocks, crafts and so on - and then figure out storage solutions for each. Outfit some of your shelves with bins and baskets for smaller items, and leave others open for books and games. Baskets, boxes and galvanized metal buckets also have the benefit of being portable. When possible, label them or have them personalized, or color-code them by activity or by child.
Consider setting up zones for specific activities: establish a dress-up area with a basket for clothing and wall pegs for hats and accessories.
Create a craft corner with a table and containers to hold drawing and painting supplies.
A few large, flat boxes can hold finished masterpieces. With storage organized by activity, kids know exactly where to look for things and where to put them back.
Look for furniture with storage built in. Put overlooked areas to work: hang buckets on the wall for small items, and put hooks on the backs of doors. From time to time, rotate toys from closed to open storage so kids always have "new" things to play with.