how do you keep a job and use?
Seriously? At least at my job, and I think this is quite typical of a desk job, you get a pre-employment screen and that's it. We don't do randoms unless you operate machinery. If you can't get clean to pass one test that you know is coming, well then, I guess you don't get hired in the first place.
As far as keeping a job, I don't get tested, don't intend on getting tested, and feel that "using" pot has absolutely zero effect on my work performance.