It sounds like an obvious answer, but the best way to produce clean urine for a drug test is to not take drugs in the first place. Of course, there are a magnitude of different methods commonly used to try to cheat on a drug test, but it's highly doubtful that most of these would work.
None the less, here are a couple of the more common methods people use to try to trick their way through a drug test.
There are loads of websites that sell detox drinks specially designed for those who want to try to cheat on a drug test. An example is Urine Luck (get it?) that sells drinks such as Absolute De-Tox Carbo Drink. It even comes in a variety of appealing flavours for the more picky drug test cheaters out there!
Here's how it's supposed to work. Absolute De-Tox Carbo drink claims to hold the drug toxins within your body, and as such, none of them pass when you urinate. Sounds simple enough, but it's unlikely to work.
Another "how to pass a drug test" myth is the consumption of baking soda. The idea behind it is that the baking soda is mixed with water and drank. When the baking soda travels along the inner workings of the body, it's supposed to remove and trace of drug use with it. I suppose it's a bit like how drain cleaner is supposed to work.
Of course in reality, this certainly doesn't work. The body is far more complex than a set of tubes that can be easily cleared. It's actually pretty dangerous to consume a large amount of baking soda like this, so I recommend that you don't even think about trying it.
Here's a video claiming to document some facts and myths about passing a drug test: