Most of the time you do not have to worry about a publisher taking your work. It isn't something they are looking to do. Everyone things about this, but publishers get 1000s of manuscripts. It just doesn't work in a way where you have to worry about it, but if you are really worried about a neat trick is to put it in the mail and mail it to yourself. When you get in back in the mail, do not open it. You will then have proof (a time and date from a federal office) that shows you owned it long before anyone else.