This is perhaps my most valuable tip: WRITE & READ EVERY DAY!
I believe the most useful thing a writer can do to improve their craft is to practice it daily. Sitting under a tree thinking of plot structure and character development is time well spent but it doesn’t get the baby washed. Writers write. Serious writers write every day.
The act of writing will develop your voice and style, even if you’re writing garbage. And yes, allow yourself to write garbage because that is far better than not writing. The more you write, the better your garbage gets.
I feel that it’s also important to establish a routine, say, write two hours every morning, or two hours before bed. Setting aside a certain time of day trains your mind when to jump into that creative mode. It becomes automatic, or at least easier. I prefer mornings, 7am to 11am. I sometimes push it into the afternoon if I’m in a groove.
I know some published writers who try to squeeze in writing whenever they have a few moments, or a spare hour. I honestly don’t see how they manage to get anything done. My feeling is, if you’re serious about your craft, you make the time, even if you have to wake yourself a few hours early and go without sleep, or miss your favorite TV shows. If you are not serious about your craft, why bother?
Excellence is won by training and habituation. We are what we repeatedly do. Excellence, then, is not an act but a habit. – Aristotle
The second most valuable thing a writer can do is read daily. Read a book a week. Study how other authors handle character development, plot structure, sentence structures. There is a wealth of fantastic examples out there waiting to teach you, and all you have to do is pick them up and read. Read everything – dead authors, really dead authors, live authors, fiction, memoirs, biographies. It’s important to read tons in your own genre, but it’s equally important to branch out.
Pick the best of breed in each genre, which doesn’t mean the best sellers. It takes a bit of work to find the really fine writers. When you come across an author that strikes a cord within you, read everything they’ve written. Good writing touches peoples’ inner feelings. If you find a writer who can do that to you, study him/her.