There are a few ways to approach this... all of which have been discussed here, so I will summarize:
1) Dilute your urine by drinking water - You'll dilute the cannabanoid metabolites to a level low enough to not get a positive, but you also dilute the other metabolites to a level where the test is considered invalid. This is probably okay, they will just make you go back another day and do it again. One lab I went to reported the test as "inconclusive". This was a pain, since I hadn't even been smoking. Figures.
2) Take a cover up - Some products you consume just hours before the urine test. The process is to dilute your urine enough to flush out the metabolites, but the cover up adds in the other metablolites to give you a valid test result. These products contain all types of vitamines, including B12 and B6, as well as creatine. From my experience, these products work very well if you want to pass a test by peeing in a cup.
3) Get fake urine - You can use either someone elses urine or synthetic urine. The two issues with this approach are 1) delivering the urine without being caught and 2) having the sample be the correct temperature. There will be a thermometer in the cup which will say how warm the sample is. If you go in to a bathroom alone, you can put the sample in to a cup and then run hot water on the outside of the cup to raise the sample temperature. If you are monitored during the drop, there are devices to warm the sample, operated by battery. This is a sure-fire pass, but takes the most commitment and preparation.
4) Don't smoke weed. It had to be said. If you have 30 days to clean out, do it. It's not a bad thing.
Vinegar, Exercise and Certo -
I won't comment on vinegar, as I have no first hand experience. Extreme exercise are probably not a good idea. In fact, exercising days leading up to a drop is definately a bad idea! It releases metabolites in to your urine. You want as few as possible. So the days up to the exam should be sedintary. You may even consider eating fattening foods to make sure you are in a "adding fat" state, rather than burning fat and releasing metabolites. Certo does work and it does so by coating the lining of your excratory system with a thin coat of wax. Water essentially goes right through you and no metabolites pass from your body in to the sample. I know it works, but it seems extreme to me and there is probably a right and wrong way to do it.
Flushing with large amounts of water days up to the drop is pointless. Flushing for 12 hours prior is all that's necessary. After the first gallon, it's practically pure water anyways.
My recommendation:
Get a cover up and follow the instructions exactly. I have used them a number of times and always passed. I passed federal examinations as well as private lab examinations. They are simple to use, and usually require only 2-4 hours advanced notice, so you can keep one in your car and buy yourself the time to pass at a moments notice. Just remember to follow the instructions to the letter. Most of these kits require you to drink very little water, which may be counterintuitive, but it works.