Prepare for rejection. Expect it. Often you will get nothing more than a form letter along with your rejections. If you're lucky, you'll get a few lines of constructive criticism. Writing is a lot like dating, you have to kiss a lot of frogs before you find your prince.
People will tell you a story is terrible. Don't storm off. Ask them why they don't like it and what they would do to change it. Don't get me wrong, if you believe in something, REALLY believe in it, don't change a thing. But think of rejection as a chance to polish your story. Or submit somewhere else and be prepared for possible rejection again. Even the best writers spent years being rejected before someone stood up and took notice.