Place a rubber pad on top of the batter head (the main head of the drum) to reduce the volume of the drum.
Replace your old drum heads with thicker heads. Thicker heads will reduce the sound of the drum.
Tune the drum down from its regular pitch, by loosening the screws around the side. Don't loosen them so much that the drum rattles. Making the pitch slightly lower will make it possible to decrease the sound naturally.
Keep your sticks closer to the drum and choose thinner sticks to reduce the volume of the drum.