If you are going to be observed, you could try a detox drink. These sometimes work, but not always. They can cause the sample to be dilute, have low creatine, and low specific gravity, which may count as a failure. I failed using one of these drinks. It was supposedly the best one on the market. The best way is substitution. I have passed several tests this way.
If they insist on observing you, you have several options to stop them. Your best bet is to say you are homosexual and that you would feel extremely violated having a same-sex observer (note: they CANNOT send opposite-sex observers due to sexual harrassment issues). Accuse them of trying to sexually harrass you by having someone watch. If you make the observer feel awkward, they will not go into the bathroom with you. Also, you can say you have really bad diarrhea and you don't want them to smell it (that will freak them out).
Once you have the observer thing out of the way, the rest is easy. Before your test, get someone that you know FOR SURE is clean, and who DOES NOT take prescription medications. Get them to pee in a bottle. Refrigerate it, because it will rot after 18 hours. Wash out a condom, and pour the urine into it, tying a knot at the top. All you need is 60 mL, so that is about 1/3 of the condom. Right before your test, get a bowl of very hot water. Leave the condom in the hot water so that it gets to be about 110 degrees. It is very important that the sample is 92-100 degrees, or they will reject it. Wear 2 pairs of tight underwear and jeans or sweatpants to keep in the heat. Put the tied off condom down the front of your pants and make sure that it will securely stay there with no bulge. If you are worried about it staying warm, you can also put a hand warmer in your pants. Put a small swiss army knife in your pocket, or some other type of scissors/knife, as pricking it with a pin will not work. When you go into the bathroom, cut the tip of the condom and pour it into the cup. Make sure the temperature strip reads that it is warm enough. If not, hold the hand warmer on it for longer.
After this, you're good to go. If it weren't for this method, I would have been kicked out of my university a long time ago. They give the most advanced test, and this is the only way to beat it. Good luck and keep smokin! Happy 420