The general strategy for passing urine tests is to increase intake of fluids and flow of urine. This helps in diluting the concentration of drugs (such as marijuana) in the sample below the threshold of detection. It is important for you to fill your bladder with fluids an hour or two before the test. The amount of fluid should be as much as you can drink.
You may even opt for a large dose (50-100 mg) of Vitamin B2, which is available in B-complex multivitamins. This will help in coloring the urine yellow, since otherwise your body is likely to produce clean and watery urine that adds on to suspicions. It is worthwhile to note that Vitamin C does not help in passing marijuana drug tests.
If you are about to take a marijuana drug test and notified of the test in advance, you can even use weak diuretics such as coffee, cranberry juice, certain health food products, and over-the-counter pills for pre-menstrual water retention. If the test is to be taken by you on a short notice, it is recommended that you use more potent diuretics such as furosemide (Lasix®) but DO NOT use it if you are diabetic or pregnant. In the case of an emergency, you may try adding a few drops of Visine®, detergent, bleach, or salt to the sample but this may be detected by smell or visual inspection.
It is very important for you to note that you should insist for a urine test if you have smoked marijuana recently and a blood test if you have used marijuana in the last few days but are not currently under the influence. You should also remember that urine sample does not turn positive until several minutes after smoking.
If you are asked to undergo hair testing, under extreme circumstances, it is recommended that you treat hair with Head & Shoulders® (Fine/Oily), Neutrogena®, and Rave® rather than Pert® (Oily/Fine) or Prell® (Normal, Normal/Oily), as this has been certified by one test though finding of the test cannot be completely relied upon.