There is really only one solution I can think of: build a giant closet downstairs. Here is the extent of our storage: 1 coat closet, 1 walk-in closet in the master bedroom, and 1 closet in the guest bedroom. That's it. 3 closets. We put up some shelving in the garage, but that's really for "outside" things, like gardening utensils and car washing stuff. I've had to resort to putting some bins of holiday decorations out there, which certainly isn't ideal.
So I'm thinking that we should put a closet in one of two places in the downstairs:
Hm... well, those pictures didn't quite turn out as good as I had hoped they would. But you get the idea. I don't know which option is best. The first option, which would take away our music nook, would certainly give us the bigger closet. The only problem there is the window. The closet would have to be a silly shape in order to avoid the window. The second option would be a bit smaller, but would get rid of the somewhat useless space behind the downstairs door. But we would also have to put a light in the second closet option.
This won't be a cheap fix to our problem, but will certainly be worth it. We'll be putting money away each month to pay for this closet until we have enough to cover the cost.