As a Fashion Photographer for 20+ years, I have come to appreciate the importance of some very important items that one should never be without. I have based this list on items that I wish I had for several of my shoots, but were not on hand and would have helped had I had them.
Although some of the items are seemingly obvious, they are often overlooked, thus the reason for this list. Feel free to add some of your own.
1. Gaffers Tape
2. Wood Clothing Clips (used for styling clothing, clipping gels to light stands, etc.)
3. Neutral density filter gelatin (for hot lights and flash as well as the lens)
4. Black Cloth 1 yard/meter square (for controlling light in so many ways)
5. 18 percent gray card (for those of you not using a light meter on the set great for metering from in camera)
6. Fishing cable. (great for using as a support between light stands, to suspend objects, etc)
7. Rubber Air Blower (preferably the ones from a Pharmacy, much better than the lens type)
8. Penlight Flash Light and extra batteries
9. Extra SD or Compact Flash Cards
10. Gold Foil survival blanket. Great for keeping the model warm between clothing changes, as a reflector and keeping one dry during a rain storm.
If you have any more you wish to add, just write to firstname.lastname@example.org