Letters of recomendation: It's easier for the writers to send them to AADSAS and much easier to keep track of. I think it's better if all your letters are enthusiastic, so don't send a so-so letter out to a school that doesn't need it. If you have a so-so letter that is only required by one school, then send it separately.
DAT: I took it August 2nd. If you're unsure about how you'll do and think you may want to take it again, do it sooner because you have to wait 3 months in between and that may hold up your application review. Your chances of getting into school decrease significantly the later your application is completed.
Essay: You can do an internet search and find tons of advice for writing a good essay. I had a family member and science professor look mine over. I also spent about two months on it. My advice? I'm glad you asked. Focus on your real qualities and reasons for going into dentistry (and then ham it up a bit). I think it's a mistake to write what you think they want to read. I'm sure they get tired of the same old B.S. from 9 out of 10 essays that all sound the same.
