Do you have a critique group of other writers that can give you honest feedback on your stories?
What is it about one work that makes it better than another. Figure that out, then make the bad one better.
Sometimes - rarely - my first pass at a story is really, really good. That doesn't mean it's perfect, but it can be almost as close to perfect as I can make it.
Usually, my first draft is crap. I decide if it's pure crap or crap with potential. If it has potential, I re-write. Sometimes I have to go over it several times to get it ready for human consumption.
Then, I submit it to my critique group. That's the only way I really know if my story is up to snuff or not. They will tell me if it's publication ready, or if I need more polish.
By the time I'm on submission with a story, the quality is fairly consistant.