Discovered via Merlin Mann of 43 Folders, productivity fame. Here they are in plain text so you can copy and paste and place somewhere for constant reminding:

1. Work on one thing at a time until finished.
2. Start no more new books, add no more new material to "Black Spring."
3. Don't be nervous. Work calmly, joyously, recklessly on whatever is in hand.
4. Work according to Program and not according to mood.
5. When you can't create you can work.
6. Cement a little every day, rather than add new fertilizers.
7. Keep human! See people, go places, drink if you feel like it.
8. Don't be a draught-horse! Work with pleasure only.
9. Discard the Program when you feel like it - but go back to it the next day. Concentrate. Narrow down. Exclude. 
10. Forget the books you want to write. Think only of the book you are writing.
11. Write first and always. Painting, music, friends, cinema, all these come afterwards.

The commandments apply to more than just writing. 

For more by Mann check out his podcast Back to Work, and two classic posts on his blog, Better and Cranking.