Tweaks when building a listening room?

Don't allow any 90 degree corners. Use dry wall to enclose all the corners to make bass traping. Use the new sheet rock called Quiet Rock I believe. You won't have to double it probably. Use rock wool as insulation. Wire at least one home run per component and then add more. Install a separate electric panel for your room where all audio/video circuits are located.
Build in the design a wood floor. If concrete, build a wood floor over it with 2 by 4 laid flat, then plywood, then carpet.
Use Cardas Golden Ratio Data to determine the best size room for your needs.
Hire Rives Audio for overall room design. As for book cases being diffusors and Traps, go to Real Traps.com and watch the video on diffusion. In fact it would really help you to view all their videos. While you're at it go to the ASC site, readytrap site, and rsc. sites.
I could add more.

I built my room about 3 years ago. If I could build it again, I'd include the ideas I've outlined here. I've learned so much since I built my room originally.

Have fun,
Steve

The flexible conduit mentioned above is a must. You always want the chance to pull wires. also, run an empty conduit with a string inside. you will be amazed how soon you will be using it.

On the walls, I would use two layers of sheetrock, but of different widths. I would probably put 3/8 against the studs and then use that newer fiberglass-based mold resistant sheetrock as the outer layer. This stuff also does not vibrate like regular sheetrock. I stumbled on this fact when I mounted some in-wall speakers in both kinds of sheetrock and the fiberglass stuff did not vibrate as much when I touched it while playing music. One other thing you might do is tack a layer of accoustic rubber between each layer of sheetrock. This stuff stops soundwaves really well.

If you want the room to sound entirely neutral, avoid drywall entirely. Sound bounces off drywall, distorting the music as it reaches your listening position.

There are sound-absorbing wall materials that can render the room anechoic, and this will enable you to hear your system without participation by the room. You can turn your system up as loud as it will go, and you will be amazed at what it sounds like without echo in the room.

However, these wall materials can be difficult to work with, and may not be aesthetically pleasing, so you will also need to figure out some means of covering them that does not then bring back the phenomenon of sound bouncing off walls. If fabric wall covering can work for you, that might be best.

Merely putting the sound absorption material inside the walls, and covering them with drywall, will not do much good.

The number one rule is "broadband absorption' - keep the acoustic treatments even. If you have floor to floor carpet then you may already be ok in the mids and treble from the get go. Then all you need is large thick bass broadband absorbers and plenty of plush furniture. See my virtual system example for what you can do with a fireplace. There is such as thing as too much mid and treble absorption - so keeping the room lively may actually make it sound better. It is pretty much impossible to overdo treatments in the bass - so do as much as you can aesthetically handle...

Whatever you do, don't try to make the room anechoic. Anechoic rooms are not intended as music listening rooms.