I will pass on what I learned as I went throught the medical last year:
1) Do not lie about past use of weed. The official outlook on past use is "We understand that people sometimes make bad decisions." They need to hear from you that you are through that stage of your life now. You may want to settle on exactly when it was that you stopped using - it helps to be able to say, "I haven‘t lit up for two and a half years now."
If you have delved into acid and other hallucinogens, you may have trouble getting in. I‘m not sure of the time constraints about how long ago it was since you used that they‘ll look at you. I think the military recognizes a big jump between a few joints and dropping acid.
2) The urine test is to test your blood sugar to determine if you are diabetic. Nothing else. That I was told right from the MO who was doing the test on me. You don‘t need to worry about false readings.
3) If you lie during the process, and it is discovered, you‘re screwed. The information you sign to as true on the form is basically a sworn legal document. If the military can‘t trust you to tell the truth, they really have to question if you can be trusted with the life of a fellow soldier. This is a big issue for them, far more so than a checkered past as a youngster. I actually knew someone who had passed the qualifications to be accepted as a pilot trainee, and it was then discovered that he had lied about using pot in the past. "Don‘t let the door hit you in the *** on the way out." Again, it wasn‘t the usage - it was that he had lied.
The military is one of the last places in the western world where you will find situations where your word is your bond. Break that, and you lose a lot.
Hope this little ramble helped. Good luck.