I've been a cop for seven years, so here's my bit of advice (which turned into a dissertation)...hope it helps. I work for a midsize agency (app. 1600 officers) so they basically test for the common drugs of abuse (ie. cocaine, mj, etc...). Testing for anabolics is something of a specific nature that would be cost prohibitive when testing two hundred guys for each open position, so as far as the chemical part of the process I don't think you need to sweat it too much. As far as the polygraph, this is what I would recommend.
Putting aside our personal views on the subject, AS are illegal in the U.S. If you admit to using them regularly you may get disqualified. (Although I hope it works out for ItalMonster). Make a decision before you enter the polygraph room if you are going to categorically deny or admit. Before the actual test you will have a long interview with the polygraph tech. The polygraph tech is most likely going to be a veteran detective, who has spent years talking guys into admitting shit that sent them to jail for years. If you try and play games with him, he will see through your b.s. in a second. Either complete denial or admit openly (ie. forget about Mexico use, one time in high school or anything else like that...AS are not recreational drugs where you would just try them to see what happens or out of peer pressure...they know this). After the interview he will administer the test. The test will be approximately ten questions, all requiring a yes or no answer. Before the ten questions he will ask you three, maybe four simple questions(name, addr, etc). These are questions which will norm you and provide them a base to judge your later responses against. Everyone who goes in the room is gonna be nervous, no matter if he's lying or not. The technique you might wanna use other guys have touched on already. Basically you want to raise the tension in your body during the "norming" questions and and remain as calm as possible during the other questions. I would try restricting the anal sphincter over the tac method simply because if your somehow caught with that tac you are completely out of gas. They will know what your doing and you can forget about being the police in that jurisdiction. Believe me most guys get slammed before they even get to the machine because during the interview they try and b.s. the poly tech and he takes their lies and forces them into trying to give facts based on those lies which is impossible. That's why I said your better off deciding ahead of time for a blanket denial or not. One last thing...if you do deny, look him straight in the eye and do not flinch. He's looking for body language during that interview also. Sorry I got so long winded but I'm trying to give you as complete a picture as I can.