I think if you really want the flush to work, you probably should not drink water after starting the epsom salt. The magnesium probably needs to be concentrated, either in your stomach or your blood (I'm not sure which), to dilate your bile ducts. If you dilute it with more water, it might not work.

I drink water like a fish earlier in the day before starting the flush, as a head start, until my urine runs clear and often. Even so, I get somewhat dehydrated by morning, and that's normal. If you started out without getting well hydrated first, you might be in for a little bit rougher trip, i.e. a little more dehydrated and ill-feeling in the morning. You can always abort and try again another day if you already feel bad.

I heard if you drank water and it got mixed with the olive oil, (assuming you're doing the juice, olive oil flush) that the water would force the olive oil to the top of the stomach and cause major nausea. Just thought you would like to know.