Do your homework. Read both inside and outside the genre you’re writing in. Keep an eye on the current market, and figure out where your book fits in with the current trends.
Network with other authors. Either on social media or at events, make an effort to reach out to authors whether their work is similar to yours or not. You’ll make contacts, and learn a lot about the life of a published author. Going to events is another must, you’ll often find like-minded writers, and may bump into an agent or two!
Get some distance. Write it down, walk away for a few weeks and then come back to your manuscript with fresh eyes. If by the time you’ve finished your book there is a sentence that has never been tweaked, polished and edited then there is probably something wrong with it.
Know the difference between a blurb and a synopsis. The blurb is what ends up on the back of the finished product, something to entice the reader. On the other hand, publishers need to have a full synopsis of the plot, we need the spoilers to know where the plot is going. The 6,000 word sample is just the beginning. By all means submit your dream blurb, too, but ensure the synopsis is prioritised.