Aggressive editing, with an eye to intention, is the single biggest tool I use for improving my own writing. After I write a first draft, I review my draft with an eye for whether different stylistic choices would strengthen my argument.
First, I consider the flow of my argument. Does it follow a logical structure, where each step is adequately justified? Is supporting evidence provided at the most advantageous moments?
Then, I consider my stylistic choices on a number of levels: namely, word choice, sentence type, paragraph-level flow, and work-level flow. My goal is to express my argument in the most concise and persuasive way possible, and I review my stylistic aspects to see if they are supporting that goal. For example, I may have a paragraph where every sentence has the same structure (e.g., they're all only one clause). In that case, I would try to vary the sentence structure in order to make the reading experience more engaging.
Finally, I review the text with an eye to removing extraneous words or sentences. I often begin that process feeling like there's nothing I can remove, but inevitably end up removing a substantial amount of unnecessary words and phrases. The more concise you are, the more persuasive your writing will be. It's that simple.
In terms of resources, I'd recommend Strunk and White's The Elements of Style. It's a classic for a reason. "Elementary Principles of Composition" and "An Approach to Style" are particularly valuable sections of the text.