Disclaimer: I am still a predent, but this is a topic that stresses me out just thinking about it-- getting a job to pay back all those loans I will have. I was thinking that, in the spring of being a D3, I would starting researching offices in the area(s) that you are interesting in working. By starting early and doing a little bit of work ahead of time, it would save potentially weeks of research later. Starting ranking these offices in terms of most desired to least desired, based of what whatever criteria you want. It will only cost you a stamp or a phone call to apply, so mine would be a lengthy list. And then, by the time I was a senior, I would a least have a list of places to apply. And isn't that the hardest part? Knowing where to start?
Then, starting mailing your resume in staggered batches 3-4 months before you want to start working. The first batch being your dream/dreamish jobs, then nice jobs, then okay jobs, etc. Keep track in an Excel sheet when you applied, date resume mailed, any follow up contact, office phone number and address, etc. If you end up applying to 100 places, you will start getting confused about what is where. Hopefully you get some promising interviews and a conditional hire before you get down to your "crappy job" batches.
And as far as having to pass the board-- I think its a risk you are gonna have to take and hope/pray/study enough so you pass. If you want until after you pass and THEN start looking, it could be a few months after graduation before you have a job. But as a dental student, offices and their dentists know that there is a possibility you might not pass, but they still interviewed you anyway. But, if you don't pass, then you will have to wait a few months anyway then if you waited to pass and then applied. Its just a risk you gotta take and, yeah, it would be really embarrassing to 'fess up. Call it a life experience and move on.
Good luck!