The Intelligent but Ignorant Reader
Here is the most important principle of writing: To write well, you must be able to see your writing through the reader's eyes. If you only see it with your eyes, you will allow your brain to fill in gaps and ignore inconsistencies. Your brain and your words will work together to create an apparently coherent and cohesive paper. But the reader, for better or worse, doesn't have your brain and cannot use your brain to correct for sloppy writing and thinking.
When you revise, pretend you are the reader. Often you will not know who will read your paper and so you must guess who the reader is. If you are trying to honestly communicate with your reader, always assume that your reader is intelligent (at least as smart as you) and always assume that your reader is ignorant. By ignorant, I mean that he or she 'wasn't there' and cannot fill any gaps in information or reasoning.