You’ll end up buying more than you want.
Make plans when you’re fresh, not at the end of the day when you’re exhausted.
Set goals when you’re on a roll; don’t undermine them after a failure.
The hard part isn’t making these rules. As Seth Godin says, “the discipline is in obeying the rule you set when you were in a different mood than you are now. That’s what makes it a rule as opposed to a guideline.”