Under and in back.

Beneath the bathroom sink, that is.

Oh, come on it won’t be that bad.  Really.
Think of it as an excavation.  An archaeological dig of sorts.  Down through layers and other eras.
Each half empty bottle, tried once and abandoned container a relic of some past life or moment of retail weakness.

Take everything out.  Yes, everything.  Have your garbage and recycle cans at the ready.
Toss the dried up, the gloppy, the not quite right color, the expired, the left over and the enviromentally incorrect.

Sort the keepers like with like.
Decide how many kinds of hair product do you really need.  
How many half bags of cotton balls are enough?
Do you travel enough to warrant keeping ten travel size bottles of shampoo?  (that you don’t even really like?)
Is under the sink the best place to store your ___________?

Whew.  That wasn’t so horrible and the amount of things you’ve kept will actually fit back under there, without piling it four high or six deep.

If you want to use containers to help keep like things together, or some stacking baskets to take advantage of the vertical space or maybe fill a small caddy with cleaning supplies, here’s your chance.
If you don’t have things around the house you can repurpose and you decide to purchase some helpful containers-measure what you’re hoping to store as well as the space you want the container to fit. Armed with that information you can shop knowing that what you buy will help, not add frustration.

Under the sink is valuable real estate in your bathroom.  
Don’t let it become a burial ground for stuff you don’t need, products you don’t use or supplies that should live some where else.

Oh, and I will be checking under there next time I’m at your house….
(No. I won’t.)