Get started on your papers early. The sooner you starting getting your concepts organized, the better you will do. Most schools have a writing center that will help you proof read, and make suggestions on parts of your paper that can stand for improvement. Use these places to your advantage. Writing comes pretty naturally to me, but I still use these places. They catch things that I would otherwise just graze right over, or they see a sentence slightly differently than I do. It can make a big difference.
As others have said, do the reading. A lot of people feel like the reading in an English/writing class is irrelevant, in reality it is anything but. You will find all kinds of tips and suggestions on structuring a paper. Reading about things like the different methods of outlining and organizing can be very beneficial. Reading about a professionals perspective on how to best structure an argument can make all the difference in the world. For instance, knowing where to place your strongest point in an argument can make all the difference in the overall effectiveness of the argument. A lot of the time, if you are using an argument with 3 main points, it will be best to structure your argument with the medium strength point first, then the weakest point second, then the strongest point last. In other situations, you might want to use your strongest point first, and repeat it/reflect on it throughout the paper. The reading will have a lot of information on what type of structure will work best in certain situations. Additionally, reading about the Aristotelian Triad (Pathos, Ethos, Logos) which is included in most English composition courses, will improve anyone's ability to form a strong argument.