Hola, doc yardener here. I work in psych as a psychopharmacologist and have given lectures to MDs, nurses, etc about how drug tests work and how people cheat them.
Peoples comments thus far are pretty spot on. Some thoughts
1. Urine drug screens (UDS) are fairly easy to beat, as a positive response only means you've exceeded a concentration thereshold (i.e. 50 micrograms/ml for MJ). So, the trick to beat them is either A) have no MJ metabolites in your system, or B) dilute your sample so the concentration falls below the cutoff for a positive.
2. UDS's validate a sample on the following criteria:
a. Temperature - needs to be body temp, so they know you didn't add tapwater or use someone's old, cold piss.
b. Ph - If the sample is overly dilute the pH will be off (not acidic enough). That's where the cranberry juice comes in.
c. Creatinine - Just a measure of muscle protein (serum creatinine) in your urine. If it's super low, that's another sign of dilution. That's where creatine supplements help, or just eat lots of animal protein in the preceeding days.
d. Color - If it's not yellow enough, that's another sign of dilution. That's where B-complex vitamins come in - they turn your piss orange-y.
So...
your 1st piss of the day will have the highest concention of MJ metabolites. So, clear the chamber that AM before your test. And drink lots of cranberry juice up until your test - for the fluid dilution + acidity. Take a double dose of b-complex vitamin that morning. If your test is around noon, you'll be okay Ph and color-wise.
All the kits they sell are pretty much bullshit - they're just herbal diueretics and all the directions include "drink lots of water." MJ is a fat soluble drug, so you can't flush it out with water. That's also why it lasts so much longer than cocaine or other more water soluble drugs. How long MJ lasts in your fatty tissue depends on your metabolism and body fat %. But, for an in shape guy, casual use lasts maybe a week or less, while chronic smokers it can last a month or more.
Real drug tests are by definition supervised, so be careful about plans including fake piss or a whizinator or whatever. If you've just been chippin at the bowl here and there at like a gram/week, and you aren't a fat fuck, you should be okay w/ the dilution method.
and BTW - a known diluted sample = FAIL. If they really want to hire you, they might have you repeat it, but typically, if the sample fails color, pH, etc then it's considered a failed test and your screwed.
Good luck!