I've had to take countless drug tests because of probation, and I would quit smoking weed for about 2 weeks and drink lots of water over that time, and you will pass your drug test. I would recommend drinking about 1 liter of water and pissing a couple of times before you go take your test. Just quit smoking now and you should be fine. THC sticks to your fatty cells in your body, so if your are skinny you will metabolize the THC in your system faster than someone who is fat. I'm in fairly good shape, so it doesn't take me 30 days to get clean from weed. I would say 2 weeks and I am good w/o even trying to beat the test by drinking water and shit. Everyone is different in this aspect. I hope this helps somewhat.
If you do decide to drink a bunch of water the day of your drug test, just make sure you don't drink to much because you can dilute it to where they can't test it. I would say not to drink more than a 2 liter of water the day of your test. Don't use that test clean shit because I know alot of people who it didn't work on.