Ok, I will try my best to address your issue.  First off, I as a prior member on many oral boards, often felt that 21 was too early to embark on a career in law enforcement.  Many and I do mean many young men lack the proper maturity to work in this profession at that age.  So, I personally would have more than likely looked at the failure to provide proper notice in prior jobs as a BIG negative on your part.  Also, negative information from your current employer WILL always look badly.  Yes, we do understand that not all bosses like their employees, but you also have to understand that we are entrusting you with a GUN and a BADGE and a lot of responsibility, so often times we will more than likely take the employers remarks at face value, but compiled with other things will begin to add more credibility to them.

The drug issue, is really a non issue.  We never cared if you lied to another department, so long as you were truthful to us.  We did however, look over your information more closely and grill these applicants more harshly because they do actually have a history of lying on applications.  One issue that may be a problem is if you were in fact over 18 when you used the drugs and also time since last usage.  My experience is that they usually wanted 5 years since last use.

As far as protecting yourself, well just be honest with the investigator and on the application.  Under the section where it says may we contact this employer or reason for leaving put it down that you and that supervisor had problems and explain it.

I wish you luck in your endeavor and hopefully in your career in law enforcement.  I really can't see myself doing any other kind of job.  Also, keep applying and trying, many good officers have been turned down by one department or another.