Give yourself time to re-read your work before sending it out.
See earlier tip about how I missed a mistake on one of my stories. Re-reading can remind you that you did write a good tale. Equally a gap between writing it and reading it can help you see ways of fine-tuning your tale to make it even better.
Read inside and outside your genre(s).
Writers love reading, it’s why we write, we want to add to the great grand canon of books and stories out there. Reading outside your genre can inspire you and ensure you don’t just read, and therefore just write, one kind of thing. It helps keep your writing fresh.
Ensure you follow presentational rules.
Or everything you do will be automatically dismissed and sent back to you. Everyone gets the thud on the mat as their work gets rejected so at least make sure yours is turned down due to publishers and not to anything you’ve failed to do, which you should’ve picked up on.
Get into the habit of writing regularly even if is just ten minutes daily.
Regular writing means you are writing. The more you write, the better you get as you find better ways of doing things to make the most of the time you do have. To become a professional writer, you need to “ape” them in the nicest possible way and they write regularly enough!