I think getting the opportunity to write you're own letter of rec is great because you control what is in the letter and also, when it gets finished! (no more sneeking peaks at letters and finding out that it was only one paragraph long! hehehe)
Anyways, One of my professors let me write my own letter. You should kinda start out by writing a bit about how you got to know the dentist (from his/her point of view of course). Then basically go over what qualities that you have that would make you a good dentist, the kind of work that you did in the office, etc. As you're writing/composing the letter, you should instill a sense of enthusiasm towards you...like writing the letter isn't a chore but rather an easy reflection on what you are like. the whole point of letters of rec is to make others who don't know you enthusiastic about wanting to meet you.
good luck!
-cusp
p.s. you should get some of the dentist's letterhead (or you could make up your own w/their approval of course) and print the letter on that.