If you are not within the FBI's policies for employment suitability, don't apply.  You're forgetting that the polygraph is not really your main hurdle from an honesty stand-point.  If you have significant drug history even from seven years back (although you mentioned some drug usage since then), you're prior usage will likely be revealed in your background investigation even if you do pass the polygraph.

Do yourself a favor and pursue another organization that doesn't require you to be within a particular drug policy.  Don't jeopardize your suitability for later employment when you know you clearly aren't qualified for an FBI support position.

Regarding the polygraph, using countermeasures may help someone pass even if that person is being dishonest on relevant questions.  However, as concerned with this whole topic as you seem to be, I would guess that you'd most likely fail anyway.  The techniques described here and other places are meant for honest people to utilize and minimize the potential risk of being falsely accused of lying.  Continue with school and seek employment elsewhere.

I imagine it would be a lot of fun to just go into a polygraph exam with the intention of doing nothing but rattling the examiner and passing it.  If you went in not worried about passing it, you might actually have a chance of passing it, ironically enough.  However, the ramifications of not passing it don't really make the excitement of doing it worth it.

Just because you've used drugs in the past doesn't necessarily mean you will have problems with a background check.  Your drug history may not even be within the scope of the background investigation depending on the employer conducting it.   

However, you can be sure drug history will be covered by an FBI background investigator.  That investigation is the FBI's last chance (before they hire you, anyway) to confirm that what you have told them seems to be truthful.  They will reach out to any references you've provided and will also solicit names from those provided references to further network and dig deeper into your background.  This is how your drug usage may be revealed despite the fact that you were never arrested.  Hence the reason I advised that if you aren't qualified, it's a bad idea to apply.

Also keep in mind that just because you get hired by the FBI doesn't mean they won't turn around and fire you should they come across information they don't like.  Not a very satisfying job if you are constantly worrying about losing it.

You can find plenty of jobs out there that are "good," even some in federal law enforcement.  Not all agencies have drug policies, not all agencies use a polygraph for pre-employment screening.  Seek those out (to name a few - NCIS, DSS, AFOSI, Army CID).  I'm sure all of these agencies have a need for people to fill support positions just like the FBI.  Besides, if you are in a position to pursue a doctorate, you won't have any trouble finding a job later on in whatever field you are studying.  Good luck.