What worked for me at the BC in Nov. (first time I did this):
1) Keep it short/brief.
2) Fax it 3 days in advance of your arrival.
3) I added a couple of Disney clip art to my letter. In my case, I used Tigger -- my favorite character.
4) I used "Mickey" font (you can download from the Internet).
5) Prioritize your requests; list the absolute most important one first, then 2nd most important, etc.
6) Don't ask for "the moon." Sometimes asking for less will get you more.
I ended my request with "Have a Tigger-ific day!"
Result: we booked as standard room, non-concierge. What we got: standard room on the concierge floor, no extra charge. I wouldn't expect this again and I don't know if my rather silly request did anything. But we did have a wonderfully magical vacation!