mortar is generally used. Put plastic down on the floor. Put your mortar down and then layer over with another piece of plastic so the mortar don't stick to the tub and floor for future removal purposes.
A sack of quikcrete sand mix is used by some. I've heard of some using plaster of paris. Basically something to support the bottom and take the bounce out of the tub bottom. Use something even if the tub has 2by rails to support it. Keep it away from the drain though.