The way to beat a drug test is......
Thats right. You can't. I know there are people that are gonna say, "Well, I knew so and so who took fruit pectin and niacin, and drank a lake's worth of water every day, and he passed because it coated his kidneys-" or "I took a detox drink, and I passed."
Okay, well here's the simple truth. You were already clean. Let's have a little physiology lesson. For those of you out there who believe the trick is to take something that can coat your kidneys so the THC-byproduct won't show, you are horribly misinformed. You cant coat your kidneys. It's impossible. Your kidneys filter wastes and extra water from the blood, then the excess fluid and the wastes are then filtered through the nephrons, which drains down into the collection ducts, into the renal pelvis, into the ureters, and into the bladder for disposal. Notice before I said filter. What happens if you coat a filter with something? You no longer have a filter. Therefore, if it even were possible to coat your kidneys, you would soon go into renal failure due to them not being able to process things correctly. Now, I could go more in depth on that subject, but it's best to keep things simple.
Ok, next one. Drinking shit loads of water will dilute the urine and make the THC-byproduct undetectable. Ok, There are a few problems with this. One, THC is not a water-soluble agent. Anyone who has ever cooked with cannabis knows this. Two, in the instance of highly-diluted urine, most tests of any merit will show invalid, causing them to test again, or in the case of lab-run testing, they will still be able to detect the by-product by spectrometry and gas chromatography.
Now speaking of labs and what not, these people are NOT, I repeat, NOT stupid. They have seen every trick in the book. All of the "cheat" methods that people use at home, and all of the products on the market, they are able to detect in the lab, and that, my friends, will be an automatic failure for you. So please quit wasting your money on detox kits.
Exercise. Will it help? This is a very cloudy area. In some ways, it will if you're doing it correctly and burning fat, but in some ways it won't because you will always have body fat, and we all know where that THC-byproduct hides.
Now let's talk about how to be sure you'll be clean when the time comes. This isn't a cut and dry thing. Metabolism plays the biggest roll in this, but there are other factors too, like your height, your weight, your body fat percentage (sometimes), your specific genetic makeup, how often you smoke, consume, vape, etc, how much cannabis you use and the THC content of it,.... so many things. Really, the way I've done it is to stop smoking at some point, to buy a bunch of drug tests, and test myself daily, then figure out how long it takes me to piss clean. Then I will buy a very sensitive drug test to make sure I come up clean on that, too. Even that isn't cut and dry though, if you don't have a mainstay strain of cannabis. If you jump around to different strains, the variables are endless, but it should at least give you an idea of how long it takes you to be clean.
This is all risky business. The only way to know you'll be clean, is if you're in the position of having to take drug tests, quit smoking, or smoke every once in a while so the time to being clean will drastically drop. Drug tests are made quite well these days, so unless someone can develop a fat-soluble agent that can dismantle the THC-byproduct, there is no way to cheat the system.
On a side note, as you can tell by reading all of this, I'm rather scatter-brained at the moment, but not all hope is lost. Many employers out there will throw away the test before even reading it, if they see you agree to, and perform a test without hesitation and you don't act suspicious. Also, low-budget testing usually only has a 7 day window, or a 75-50ng/ml cut-off limit, so you should be fine within a short period of time, unless they have a more sensitive test. God-forbid you have to go to a lab for the test, in which you could be hot for 3 months by urine alone.
I hope this helps, legalize everywhere, and everyone have a very stony day.