Seth Godin:

Art is hard.  Selling is hard.  Writing is hard.  Making a difference is hard.  When you're doing hard work, getting rejected, failing, working it out -- this is a dumb time to make a situational decision about whether it's time for a nap or a day off or a coffee break.  Zig Ziglar taught me this twenty years ago.  Make your schedule before you start.

Poke the Box, p. 18-19