ive talked/written to many deans throughout the admission process, and here are some of my personal suggestions:
1. be concise. deans are busy and wont read through a 1-2 page letter if it doesnt grab their interest.
2. no fluff. this is related to #1. every word should have a reason for being included.
3. try to tailor your reasons of interest to the specific school, and be specific. dont include stuff like "i really like your school because of the excellent research opportunities!" Be specific as to WHICH research opportunities you are talking about, and why you could not find such opportunities at other schools.
4. Try to get other faculty members to write to the dean on your behalf. Receiving a two-sentence letter from an important faculty member endorsing you may have a lot more impact than you writing a two-page letter yourself, not to mention it is a lot easier.
Hope this helps...