...read the submission guidelines. I had to reject a full 1/5th of the stories I slushed today because of submission guidelines violations. It wastes my time and makes you look lazy. I've worked hard to get myself where I am. I expect the same of you.
...avoid using strange formatting, or odd fonts, or variations in the spacing between paragraphs, or... I could go on all day. How your manuscript LOOKS does make a difference in how I FEEL about it. If it looks neat and clean and professional, I'll treat it with a great deal more respect. The vast majority of the stories that end up being published are neat, clean, professional-looking manuscripts. People who care about their writing show it by the care they take preparing their manuscripts. Follow this link to a great article on manuscript formatting:
A Guide to Manuscript Formatting
...proofread your story with your own eyes--or better yet, someone else's. Don't rely on spelling/grammar check software. One or two missed minor errors is not a big deal. It happens to everyone--even the pros. But a lot of errors in your manuscript makes me think you don't take this writing thing as seriously as I expect you to if we're going to pay you for a story.
...don't submit stories told from a cat's POV. (OK, that ones mostly a personal preference. Mostly.) ;-)