Check the gutters, if they're overflowing they could be backing up into the wall/ceiling.
If you've got vents on the roof or a fireplace, the flashing around them is one of the prime areas for leaking. You can buy all kinds of waterproofing products to smear around them.
If it's leaking because of bad shingles, about the only temporary fix is the dreaded blue tarp.
Oh, if the water is forming paint bubbles on the ceiling, poke a hole in them to let the water out where you want and so that the sheetrock in the ceiling doesn't get saturated and come down.
Good luck!
