There are four types of tests which can detect drugs in your body. First is the most popular one is a urine test, second type is mostly done to detect drugs like cocaine; Hair Follicle test. The third type is a blood test and the last is a saliva test which can detect toxins used 3-4 days ago. If you want to pass a drug test, the first suggestion I would give you is to not use anything for a few days before the test. If you're a marijuana smoker, you should stay clean for at least a month or 25 days as marijuana remains in your system from 30-40 days. Check drug time table: to check how long does the drug take to get out of your system. You can also take an online drug test first and check if you are on the right track. There are a number of detoxication kits available in drug stores which will well prepare you for a drug test.
You can't avoid testing dirty if they really want to know what you have done. However when it comes to "pot" If you need an emergency clean for a minimum level test. You need to increase the level of acid in your system since the component there looking for is basically an oil and will be dissolved by the acid. To do this you need to over power the stomachs ability to neutralize its contents before entering the digestive track. I wont help you with that as it can have adverse effects on the body like death.
Don't be fooled by these over the counter substances that try and fool the drug tests. There are some tests that can determine if you are using anything to try and fool the test especially if your employer orders.