1. Writing poetry is good, but writing (and delivering) speeches is better. Nothing ferrets out difficult to follow, unnatural or confusing language better than getting up in front of a group and presenting it. Speeches also tend to be short-form, so they really force you to condense your ideas.
2. Writing is a multi-phase process, and each phase should be approached separately. Specifically:
2a.) The first step in good writing is to create a logical structure of your document - figure out what you want to talk about, then arrange the ideas you are going to cover into an ordered tree structure. Include questions you want to answer, any relevant research, and examples or analogies you want to cover. Personal opinions aside, Ayn Rand's The Art of Nonfiction is actually a very good guide to logical document structure.
2b.) The next step is to convert this structured document tree into prose. For this step, I suggest getting a pen and lined paper and getting away from the computer. Have a printed copy of your document tree to reference, and try to flesh it out without worrying too much about style.
2c.) Finally, transcribe your handwritten prose onto your word processor of choice, and edit for style. The basic tenets from The Element of Style are good. Favor short, direct, positive, active sentences. Focus on descriptive verbs and nouns rather than loading up on adverbs and adjectives. Try to trim unnecessary words. Beyond that, try to inject metaphor into your writing as much as possible. If you use a metaphor, try to stick with the same one for the entire paragraph. Use wordplay such as rhymes and alliterations to make important sentences more memorable.