you definitely don't want to drain the water out, you have to keep the membrane wet, if it was to dry out, then you would have to replace it, when you assembled the unit, the membrane should have been packed in a air tight bag with fluid to keep the membrane wet.
As for when not using it, don't do anything, if it was to sit for long periods of time just throw away the first few gallons when you need to use it again and then your ready for some more good r/o water.