5. Laundry Bag - Big and Small
If you're traveling long-term you'll quickly realize you'll need a laundry bag in your suitcase. Avoid the mesh ones that are so popular at all costs (the smell escapes through the mesh and stinks up your clean clothes!) Instead, get a plastic bag like this one. And bonus, it gives you a bigger and smaller bag so you can separate your under-things from your everyday clothes. 

Get it at Flight 001.