One of the best piece's of advice I've recently about writing is:
There are no real rules, only guidelines.
A lot of the time as writer's we get bogged down with "The rules" and the "do's and don'ts".
Forget all that. Write the best story you can. Because if you worry about the "shouldn't do's", you run the risk of having a stagnated story that sounds like a bad carbon copy of someone else's writing/voice; Instead of writing from your heart and with your own voice.
I've found it's hard to write when that small voice in the back of your head says - stop, that's wrong, you're doing it wrong, you should do it this way. So I've learned to kill that inner critic, doubt demon and just write.
Then I'll worry about everything else, because the most important thing is getting the story down and finished.
So what's the best piece of writing advice you've received? Or a better question - what is the one piece of advice you follow and apply to your own writing?