I love this advice for aspiring novelists from author Philip Pullman, in the interview "Philip Pullman: a life in writing":
One: work every day. Get into the habit of it. Work when you don't feel like it, when you've just broken up with your girlfriend or boyfriend, when you're feeling ill, when you've got homework to do. Put your work first. Habit is your greatest ally. Get into the habit of writing when you're young and it'll stay with you. Sixteen is a very good age to start.
Writers write. If you write, you're a writer.
Here's my favorite writing tip, from the Top 70 Writing Tips:
Make mud: generate text generously – there's more where that came from
We can put immense pressure on ourselves when we write. This is always a mistake.
Look on all the writing you do as "making mud". Be exuberant and messy.You can do a lot with your mud. Just as you can build entire houses with mud bricks, you can write articles, novels, nonfiction books, short stories, essays, memoirs – in short, you can write anything and everything, if you make the basic building material, the "mud" first.
There's not much more to the writing life than that.