Instructions
Buy containers to accommodate the size of your child's collection and the type of sorting system you have decided to adopt. Try using bags, drawer units, plastic boxes and bins, tackle boxes or bead containers and small jars for the tiniest parts. See the Resources section for a link.
Store Lego blocks by size if you're always helping him look for "a white 2 by 4 block" or a "flat yellow 2 by 3 piece," for example. This is probably the most convenient system since "builders" need blocks of specific sizes to complete their creations.
Divide by type if your child identifies the toys by their shapes. Store in categories like squares and rectangles, flat pieces, sloped blocks, pegs, windows and doors, wheels or characters, for example.
Separate by color if your child likes to design all-red buildings, for example. If the collection is large, consider sorting by size and then separate and store by color to make it even easier to locate the necessary piece.
Keep tiny Lego blocks like pegs or flat 1 by 1 plates in containers separate from larger pieces since they're difficult to find once they fall to the bottom of buckets and bins.
Stash pieces according to the sets they came with if your child collects kits designed to build specific vehicles, characters or towns, for example. Keep set instructions in the same container or in a binder where they'll be easy to locate if he wants to put one together.