Knowing any specifics about what kind of test you're getting (if temperature is an issue, if pot is definitely tested for on it, if they'll literally be watching you piss, really anything about the overall quality of the test, etc), would be really useful for figuring out what you need to do to get around it.
I'd say ask a coworker who's been tested for specifics about it, and go from there.
But yeah, drinking a ton of water, and not using the first urine of the day is probably the best basic advice really.
I have a friend who almost didn't get a job though, cause his test came back negative, but very diluted, just cause he'd been drinking so much water.