Do a search. I made a thread similar to this about a month ago. Same exact problem. It was my frist time smoking and I only took like 3 hits and had a drug test 4 days later. I pretty much stayed up the whole night before and drank a whole bottle of cranberry juice (yes, a whole bottle of REAL cranberry juice, not coctail), and worked out (to sweat). Then in the morning I ate a large breakfast.
I passed and got the job, although I declined it because working at WalMart would blow. Also, take it easy on the cranberry juice. I felt like shit after drinking it. Maybe I went a little overboard....