As a contractor I prefer 3/8 drywall on ceilings,but thats not code.The best thing to do is remove the ceiling and install additional ceiling rafters between the exsising joists and install blocking like crazy,the thicker the drywall the heavier and the more it sags!another option is go in the attic and install blocking in between the rafters cross wise and then use drywall screws to screw the drywall back up to the new blocking,thats alot of work other wise, apply a thick coat of skip trowel to the ceiling and paint it with a flat wall paint!