Go forward, do not go back. Tape over the backspace key for the entire month. Do not erase anything. Do not revise anything. Do not start again. During the month of November suspend all critical judgments.
Go here for more tips. (Last year I accidently found out about NaNoWriMo on November first, remember? BBEdit popped up to say “you have an update, download now?” and while that was going I rummaged around on the NaNoWriMo site and started typing… This year I’ve been enjoying the pre-November pep-talks.)
