Don'’t rely on the computer’s spell checker. It doesn’t pick up on everything. Mine has a bit of a thing about wanting to put in “his or her” instead of “their”!!!! What has it got against “their”?!!! Have a good dictionary to hand, always.
Vary sentence and paragraph lengths to vary pace. One continual pace is a monotone, never interesting. Watch out for favourite words and phrases creeping in - this is where a good edit comes to your aid!
Think about why you write the way you do. Can you improve it? Look for favourite phrases - they will creep into your writing unbidden! Mine do! Is your style appropriate for what you’re trying to produce? For example light, easy sentences might not be apt for a dark piece where I’d expect the words to be heavier, darker, to conjure up the right mood. Wodehouse stuck to what he knew - humorous prose - for a good reason. And his light style is perfect for it. Likewise horror writers write in a very different style appropriate for their work. So make sure your style matches.
Are you getting into the heads of your characters well enough? If someone asked you an unexpected question about any of them, could you, based on your knowledge of them, answer it? Is there enough going on in your story? Do your characters change? Do you show how and why? That is the story after all!