So this was how my letter went (I posted this in another topic):
1)X is my top choice- if admitted I will withdraw from other schools and immediately accept a seat at X.
2)Why do I love X so much? (Go into detail here- talk about specific classes/organizations/groups and how you will utilize them, etc)
3)I don't only plan to take but I will give- symbiotic relationship (you may not want to use the word "symbiotic" since i used it and it'll look fishy if joe happens to remember my letter). (How will your presence there benefit the school?)
4)Discuss anything new that wasn't addressed in your original application (i.e. won the Nobel Prize, rescued a cat from a burning tree, etc, etc)-specific accomplishments that will make your application stand out.
5)If there is any further info you need from me that would deem me worthy of admittance please don't hesitate to contact me.
This was basically how my letter went.
HTH and good luck.