As far as I know, and I did exhaustive research on this subject last year (I decided to go the same route you are going now, and eventually decided to go back being poor... but then again I don't have a family), and I have a couple of things to contribute.
Drinking a lot of water will not help you pass the test. Drinking excessive amounts of water will help you thin out your urine, but they test for this technique. Most reputable labs will test your urine for excess water, and will either count this urine as not being a suitable sample, and tell the employer as such. Many employers will count this as a positive.
Sneaking in clean urine is a suitable idea, if you knock out the kinks of it, especially trying to get the temperature right, as they do test this also. (My cup had a little thermal sticker on it that changed colors.)
I used one of those drinks that you buy in headshops. It was called Carbo-something. I drink this bottle (maybe half a liter), and filled the bottle up with water once afterward. It worked for me. I hadn't smoked a terribly large amount recently, so i don't know if I would have squeaked by anyway.
If you can get a little time, you should definitely start doing one thing-- exercising. THC is fat soluble, (which is why it stays in your body for so long, and also why we can all make exquisite cannabutter) and so the fewer fat cells you have in your body, the quicker the trip will be out of your body. Also, having a higher metabolism will chase it out better. This is the reason that medical authority will often give estimates for heavy-user THC eradication at figures as broad as 14-90 days.
I wish you the best of luck, and hope that whatever road you take treats you like a gentleman. Like I said before, I faced a similar situation once. I hope this job is a whole lot sweeter than the one I had.
S-Town