Writing is not easy. There are thousands of little rules that can slip your memory. One thing that really helps me is to continually be reading. Reading good writing is the best thing you can do to improve your own writing. You would not be able to list all the rules of English simply from reading, but you would implicitly know what is good writing and what is not. Implicit memory plays a huge role in how we speak and write words. We simply know what works and what doesn’t.
However, it is still good to have a helpful desk reference for writing. You can purchase a book containing all the English rules you would ever need from a bookstore for less than twenty dollars. For instance, in that previous sentence I don’t really know if I should write out twenty dollars or simply put “$20.” When you keep a reference book handy, or just have an active Internet connection, you can generally figure something like that out quickly. Obviously no one is ever going to be a perfect writer, so we can all continue to work on that skill.