It is amazing how Green Glue is taking the soundproofing industry by storm. Green Glue is a convenient sound-damping agent that is sandwiched between 2 layers of drywall. It dampens both the existing layer of drywall as well as the second layer that is installed over top of the first. Green Glue precludes more
We used to do the same thing using ordinary silicone sealant, which has the advantage that the screws can be removed after it dries, further decoupling the layers of sheetrock from the floor above. A higher tech (and costlier) method uses mats or pads of sound absorbant material instead of silicone.
A friend built his studio as a complete box with its own floor and ceiling. The whole affair rested atop a sill plate with a bed of silicone in between. The walls were of similar design to the room described in the URL below. He's a drummer, audio engineer and (coincidentally) an alarm installer. His design differed from the following in that it included a fully framed floor as well.