A major tenet of any good decluttering philosophy is that every item you don't get rid of must have a designated place in your home. Depending on the size of your living space (and your personal preferences), this can pose a problem. I've lived in a number of small apartments over the years, and have found that shelving units and bookcases work best for me, so I take a sort of storage-as-decoration approach: Not everything fits in a cabinet or closet; some things simply must be displayed. Here are the shelving units I love and one I don't recommend.
Cube storage is versatile
The first kind of shelf to consider is the humble cube storage block. I have a four-by-two in my kitchen and two smaller units in my closet, and I've owned numerous other configurations over the years. These are the most helpful because the dividers within the shelves let you get specific with your organizing. Instead of one long shelf for, say, all your shirts, you can put long-sleeved shirts in one cube, short-sleeved shirts in another, sweaters in a third, etc., which makes organizing a lot more efficient. You can also buy collapsible fabric drawers in a variety of colors to keep your stuff out of sight and match the theme of your space.
A plastic bookshelf is a cheap, servicable option
Another unit I really love is this simple plastic bookshelf. I have three: Two tall, four-tiered ones for seasonal clothing, and a smaller, two-tiered one I use as an entertainment center, with my TV on top, my consoles in the middle, and books on the bottom. What I like about these is you can pick them up in various colors, which makes storing things out in the open a little more tolerable. Just because you don't have adequate storage space doesn't mean your solution has to be ugly, nor does it have to mean investing in a statement piece. (If using longer bookshelves, you can add in some decorative baskets to keep items separated.)
A rolling cart can be an handy alternative to traditional shelves
Here's an unorthodox option: I once got a rolling printer cart off the free section of Craigslist, and though it was pretty ugly, it came in extremely handy in the bathroom of my old apartment because it had three shelves I could use for makeup, hair products, and shower essentials—plus a power strip. Outlets can be scarce in smaller places, so a mobile shelving unit you can also plug things in to can be a real lifesaver. Bonus: Amazon sells much nicer-looking ones than the thing I used to have.
Wire cube units are a bad idea
There is one shelf I hate, and it is the wire cube unit. Although it was slightly nicer looking than some other options I considered at the time I bought it, it barely lasted a year. The whole thing is held together with little plastic disks, and the individual cube floors sag under too much weight, making it useless for real organizing. Truly, anything would sag mine, eventually leading to a collapse and a big mess (not to mention a terrifying noise whenever it came apart). In a crafty fit, I tried reinforcing it with twine once; this did nothing but make it uglier. Avoid.
Other cool shelves to consider
In the process of scouring Amazon for the units I already have and recommend, I found the following cool options I may add to my own home—maybe they will fit your needs, too.
First, this tower that holds books (or records, or games) at staggered angles, resulting in a sharp look that maximizes your use of vertical space.
This classy-looking five-tiered unit with shelves staggered horizontally ($75.99)
This five-tiered beauty that comes with 20 side hooks for hanging items and is also completely collapsible for easy moving ($117.98)
This big, structured unit with some shelves that overlap to create different-sized spaces within the frame ($149)