Take your time with it BW, don't send it till it says all you need to say cause you probably won't get another chance. Sending off your DA letter can be a really empowering experience, might help you to draw a real line in the sand.
http://www.jehovahs-witness.com/6/65121/1.ashx
This is SNG's DA letter, one of the best I've read so far. Especially the reference to his dissolving membership with a printing company (may not be the exact words!). Anyhow, hope SNG doesn't mind me re-posting it for you:)
http://www.jehovahs-witness.com/6/87881/1463045/post.ashx#1463045This was my letter to my fam, not exactly a DA letter though.
Good luck with it!