Hassan has lots of good resources on this so I would definitely check those out, but basically you want to break your cover letter down into three sections: 1. Why the firm? 2. Why the division/role? 3. Why you? Hassan also has a good technique for structuring your paragraphs; PEAL, Point, Evidence, Analysis, Link. Make your point, then back with this up with your own personal evidence, analyse it while trying to use 3-4 data points to show impact, and then finally link this back to your main point/message. Use this format for all three sections. Correct me if any of this is wrong, but this is just what I took away from some of Hassan's advice on cover letters.