Just because you don’t want to invest in a “Closet System” doesn’t mean you can have a custom shelf or two. You would be surprised how handy it is to have an extra surface running above the top of the rack (leaving enough room for hangers to hook on and off, of course). Yes, it may be a little awkward snaking things up there, so think carefully about the size and function of what is stored. Measure carefully and use the 1$ per-cut service at Home Depot to get the exact shelf you need and install with simple metal brackets. Avoid the temptation of purchasing a floating shelf unless the items you intend to store are super light-weight.
In small apartments, suitcases often end up crowding closets. To save additional space, try nesting them one inside the other, using the most inner case for additional storage. I also find that luggage is particularly ideal for housing guest linens and extra blankets—again, things that are not used on a regular basis.
Start by categorizing closet contents as follows: To Keep, To Donate, To Mend, Convert to Rags and Throw Away. Ask yourself, if you haven’t touched it in over a year how likely is that you will? Articles like this are generally not worth holding onto.
I cannot express how great it is to have a vertical shoe rack.
A friend of mine turned me on the idea of a seasonal clothing-swap. Just keep the seasonally appropriate attire in the prime closet real estate, and the rest can be stored away in large zippered market bags and tucked out of sight. These things don’t have to be accessed for months, so why occupy valuable closet space?