Take a few minutes at the start of a writing session to decide what you’ll need. It might be files, or a reference book, or an earlier draft, or scissors, or a marking pen.
Whatever it is, get it together before you start. There’s nothing more maddening then being in the flow and realizing that something you need is missing. You get up, you go searching for it, and by the time you get back to the desk the flow has dried up.
If you write somewhere else (like a coffee shop or library, for instance) this is even more important. Keep a briefcase or bag stocked with the stuff you need. If you’re taking your laptop, make sure it’s charged and that you have a memory stick handy for backing up your work at the end of session.
The less time you have to spend on mundane things like supplies, the more you free your mind to be creative and to get lots of writing done in your session.
(For too long, time management books ignored the right side of your brain. That's been remedied in my book, "Focus: Use the power of targeted thinking to get more done." You can get it right now from Amazon or your other favorite bookseller.)