Looking back at my last five attempts, the biggest shift came when I stopped trying to sound 'literary' and just focused on the last line. I'd write the final image first—something that sticks in your teeth or gives you a chill—and then build the whole story backward to earn it. Contests get flooded with competent, forgettable prose. Judges read hundreds. They're numb. That last sentence is the only thing that wakes them up.
I also started hunting for contests with very specific, weird prompts. The narrower the prompt, the fewer entries they get. A general 'open theme' contest is a lottery. But if they want a ghost story set in a laundromat involving a single sock? Fewer people bother, and your odds improve dramatically. The money's often smaller, but winning begets more wins. It's a resume line.
Never submit anything you wrote the same week. Let it sit until you've forgotten the rhythm of the sentences, then read it aloud. The clunks announce themselves.