I don't know how many threads I've seen on this subject since I started participating on this forum.
Check out the studio building forum on Gearslutz. Real acousticians with real studio building experience participate there.
The biggest problem with drums is the low frequencies, primarilly the kick. STC ratings on sound isolation materials are built around midrange frequencies, primarilly the voice. Just about all sound isolation materials and techniques have a dramatic fall off in effectivness in the low frequencies, just where a drum practice room needs it.
What you need to do is absorb as much of the low frequencies as possible and isolate or decouple what is absorbing them from passing the sound though to the outside walls (where they become radiating diaphrams that broadcast the noise to the next room or the neighbors next door).
The most effective wall/ceiling isolation I've seen (without building a completely isolated floating room inside the existing structure) are the Green Glue isolating clips coupled to hat channel with 2-3 layers of type X fire rated (this is denser than standard grade) 5/8 drywall mounted to them. To do a typical 9x12 bedroom, this amounts to around $1000 not including the drywall. You can add a few hundred dollars of Green Glue between the drywall layers to further improve the low frequency attenuation. And we haven't even covered the floor yet. In the walls and ceiling, if done properly you will get around 55-60 dB attenuation in the upper frequencies and maybe 30 in the low end. Note that you can easily produce 100 dB with your drums so you are talking about 40-60 dB of their sound getting out. People will still know you're drumming in there.
So, let's presume that like most folks starting these "how do I 'soundproof' my bedroom so I can practice my drums" threads, someone doesn't have a couple grand to throw at the situation.
If you can't get into a very rigid walled space like a concrete basement or cinderblock shed, here is probably the most effective thing I've read on the net. Take some 5/8 sheetrock (two layers glued together with construction adhesive is better) and put some wood strips on the floor and ceiling so that you can set the sheetrock in there leaning outward on top a bit, resting against the wood strips. The rest of the sheetrock is unsupported, just hanging there floppy. Behind these you put fiberglass insulation. The floppy walls absorb lower frequencies and the insullation behind them dissipates it. If you can remove or knock large holes in the wall behind the new sheetrock, so much the better. You have increased the lossy volume that is dissipating the noise and gotten rid of the triple leaf system that conducts it better. Then anything you can do to seal things up. Tape the seams in the hanging drywall, weatherstrip the door and window. Put a plug in the window frame and so on.
At that point, putting absorbant materials inside the room to lower the internal reflections of high frequencies starts to help. But not before.
