Robin Lee Hatcher had a great tip in her "Staying Organized in the Chaos of Writing Class."
She doesn't believe in being in the mood or having a muse, but that you can train your mind to switch from left brain to right brain.
The suggestion was: write in a particular chair, at a particular time or even wear a particular hat, each time you write. It helps to turn on your creativity.
Any other suggestions for telling your mind "It's time to work."?