What's the most important writing advice you received that you failed to keep?
Sadly, it's the most important writing advice ANY writer can receive -- DON'T MISS A DEADLINE!
Seriously. As much as I harp on this in sessions and panels, I'm not above reproach. I've missed a few in my time, but the important to remember when it happens is to be upfront and honest with your publisher and bust your hump to get that story in as soon as possible. As long as your reasons are valid and you're diligently churning out the story, most publishers are more than willing to share a little bit of their wiggle room with you to get your best work. They've all been there. They know that stuff can happen to intrude. Just don't make it a habit.
Because "habitually late on work" is not a quality you want passed around from one editor to another in this small pond of people willing to hire you.