The first thing we purchased was from IKEA, although you can find these anywhere. A simple closet organizer. The uses for this has changed in the past year as my son has grown and changed. He's little now, so the top drawer holds his clothes for the week. Eventually when he's older (there are five pockets), each pocket can stand for one day.
On the top of the closet I have these adorable bins from the Dollar Tree. I created labels on the computer, printed them, laminated them, and stuck them to the mini clothes baskets. I really like this system, as it keeps all of his linens organized. That boy has a lot!
He has a dresser from IKEA and we picked up these drawer organizers from IKEA (they fit exactly into the drawers - concidence?) This has really helped to divide his onesies and swimwear by sizes.
I also put these in his bottom drawer, to divide up his shorts and pants into first two piles (shorts and pants) and then my size.
The top drawer contains another drawer organize (again, from IKEA) for all his other odds and ends.
The linen closet is another beast. I found this idea on pintrest (and then stole and adapted it for the nursery, can you tell?) Our linen closet is small, so I'm still unsure if I like this method or not.