1. Use the rule of 3's. No more than 3 points / ideas overall (if possible), no more than 3 clauses per sentence, try to use 3 examples to support each claim.
2. Write in a goal-oriented way. If you're not sure about what you have written, ask what the sentence, or page, or paragraph, etc., is doing. If you don' t have a good answer, cut it or revise it.
3. Counterintuitive: Don't get so caught up in being "concise" that you sacrifice clarity or effectiveness (or so caught up in being "clear" that you sacrifice effectiveness and concision...etc.); don't forget that the best writing often violates prescriptive rules; don't let the perfect be the enemy of the good. Often, people are better writers than they give themselves credit for, and worrying about whether a piece of writing fits into the paradigm of "clear, concise, effective" (or any other) can keep you from evaluating whether it truly does - which you can generally learn best by letting it out into the wild.

Well they got concise down...