Hellobee Boards

We have small, clear Rubbermaid tubs that fit on the top of shelf of their closet. We stuck an index card in the front of each one with sizes on it. The index card works well because I can look up at the shelf and see exactly what is in each box. In the early days we didn't distinguish boy from girl, it was just size. From time to time I would clear out the things in their drawers that they outgrew and that stuff would go into more tubs with a card that said "outgrown + size" to go to my sister's twins. Then I'd fill their drawers with the new size items. They'd often grow into things at different times but I just emptied out their drawers and boxes as needed. The drawers in the dresser were separated by boy or girl except their pj drawer because we do a lot of gender neutral pjs. So B took longer to get into 0-3 so I cleaned A's drawers out first and filled with the new size from the tub.

Now they have larger boxes we keep in a different closet that are labeled with name, size range, and season since they may be in one size over the course of several seasons now that their growth has slowed down. Now that their growth is more predictable I've purchased larger things for next year too. So I have boxes labeled like "Alice, 2T+, summer dresses." We still use the small Rubbermaids for things they don't quite fit into, but will soon/within this season. Keeping them right there in the closet keeps me from forgetting that they have those clothes ready to go and they don't need anything else (hypothetically) when they start to outgrow the things in their drawers!

We have one tub in the closet to put her current size when she outgrows them. Then another tub with the next size so I can grab what I need. I have tubs of next sizes in the basement since we have hand me downs. The outgrown sizes are in tubs down there too

We are dresser-free (hate them, DH and I don't use them either) so all LO's stuff is in her walk-in closet in hanging organizers. I put all the newborn size stuff in the hanging organizers at the front of the closet and then organized everything else according to size and stuffed it in another organizer in the back of the closet, or hung it up on hangers if it was nicer stuff. As she has grown, I have rotated each subsequent size to the front organizer and put too-small stuff into paper bags to loan to friends with younger kids.

@bluestriped bee: I'd get the big ones - long term, you'll need the space (as clothes get bigger). If they're TOO big for now, you could always stack girl clothes on one side, boy on the other, but keep the same size together.

Those vacuum storage bags also work well if you need to store in a shallower space (like under a bed) and don't have room for the big sterilite containers.

These are the sterilite containers I use - they're big but reasonable enough I can lift them once loaded with clothes:
http://www.target.com/p/sterilite-clearview-latch-storage-tote-transparent-with-purple-latch-66qt/-/A-13794509#prodSlot=dlp_medium_1_6&term=sterilite