The general timeline:
To maximize your DAT score, wait until you have had at least one year of bio., gen. chem, and Ochem. I would reccmmend taking the DAT in the Spring of the year before you plan to enter (e.g., I am entering in Fall 2002, therefore I took my DAT in May 2001).
You can file the AADSAS beginning June 1st. This should also be the year before you plan to enter (I filed mine on June 1st, 2001 for entrance in Fall 2002).
FYI...6 tips (from my personal experience) to make you a very competitive, well-rounded applicant:
1) Maintain a high GPA (3.5+ overall and science)
2) Do well on the DAT (19 and up is great)
3) APPLY EARLY!! Get the letters of rec., transcripts, etc. prepared by June 1st! I know way too many people whose applications were on hold b/c they were waiting for a professor to write a one-page letter.
4) Do research. Its great experience and interviewers love talking about it!
5) Shadow a dentist. Most schools expect you to have some knowledge of the dental profession, not just what you read in a book.
6) Have lots of extracurriculars. They will make you a much more well-rounded applicant (and person). Don't be Jo Blo pre-dent with a 4.0 GPA but no clubs, honor societies, leadership positions, etc. Your interpersonal skills will reflect this.
Anyway, I'm sure most of you are already ahead of the game on this stuff, but I just thought I would put in my $0.02. Good luck to all of you.