Review the construction threads on the site and check out what others are doing to futureproof their HTs. I have learned alot by watching what other are doing, or in hindsight wished they would have done.
I agree with most of the prior suggestions. Running 3/4" (or larger) smurf (blue flexible) conduit to all of you speaker locations would help ensure futureproofing.
The following is a list of some of the things that I'm doing to futureproof my HT. Hopefully, you find something that will help you.
I didn't want to run smurf conduit from my A/V closet (located outside the HT) to each speaker location. Instead, I pulled two 12/2 speaker cable runs to each of the 7 speaker locations. This was cheaper. It gives me six good quality speaker cables at my stage, which will allow me to bi-amp my mains and run two center channels (or, upgrade to 10.2), or bypass a bad speaker cable in the future. I have a A/T stage wall in the front so the cable positions can easily be extended to other locations behind the stage wall (I left plenty of excess cable on these runs).
The speaker cable redundancy also gives me eight speaker runs in the rear of the HT (2 at each surround location). My HT will have fabric streached over 1" linoccoustic ductliner and polyfill, so even though the current cable locations may not be optimal for 10.2 (or whatever they come up with next), I can always relocate or extend the cables under the fabric.
For the subwoofer, I ran a subwoofer cable and XLR cable to my front sub location (behind stage wall). If I put two subs up front, I can split the signal behind the stage wall. I'll have two 1 1/2" conduits under the riser in the back of the HT, which will allow me to pull additional speaker/sub cables as needed.
Like you, I'm also using double sheetrock (5/8") and GG. The walls adjacent the the living space are double stud walls (with two communicating doors). I'm going for maximum sound isolation, so I went with isolation clips on the exterior wall stud to isolate them from the house structure. The ceiling also has isolation clips connected to 7/8" DWC. Two layers of 5/8" sheetrock with GG are attached to the DFC channel. I'm shooting for a STC of 60+. My logic is, "what's the purpose of going through all the work of building the HT, if you are not going to be able to enjoy it due to the WAF (i.e., sound tramission outside the HT)".
Because I'm being completely anal on the sound isolation, my recessed lights will not penetrate the double sheetrock ceiling. Once the sheetroock is completely installed, I'm constructing a soffit (10" tall and 24" wide) around the HT. This will not only house my recessed lights, but I also plan on installing insulation in the soffit to construct a bass trap. The bottom of the soffit will be wrapped with GOM fabric, so if I need to I can always remove the fabric and install additional cables around the room via the soffit.
In addition to HDMI, CAT-5 and trigger cables, I have also installed Impact Accoustic Rapid Run cables (two, one digitial and one analog) from the A/V closet to the projector. This will allow me to change the configuration of the cables as my needs change. Finally, I installed a 1 1/2" conduit from the A/V closet to the projector.
Run several (at least three or four) CAT-5/6 runs from your stage to your A/V closet. Network cable is cheap, always pull a couple of spares. You may need them for an I/R extender, as a trigger cable for sub, drop down screen, etc. While you are at it, run some CAT-5, speaker cables and power inside your riser. You (or the next owner) may want to have powered-recliners with buttkickers. The CAT-5 can be used for game controllers, HTPC peripherals (keyboard/mouse), networking, etc.
The suggestion to install sufficient power outlets is well taken. In my HT, none of the power outlets penetrate the sheetrock. They are all installed in the columns, riser, stage or surface mounted behind the stage wall or inside the soffit. I went surface mounted (in areas where the outlet is hidden) to maintain the intgrity of the the double sheetrock walls. The only penetration is a small hole for the electric cable, which will be excessively chalked. (I'm in the middle of the sheetrocking phase right now). I've run two Panamax outlets (one behind the stage wall for the powered sub and the other above the projector in the soffit) back to the UPS/power conditioner system in the A/V closet. Hopefully this will prevent ground loops and will also give me greater control over the power to these units. I also placed a surface mounted outlet on the ceiling behind the stage wall (future drapes, star ceiling, who knows). Keep all of your eletrical wires at least 6"-12" away from your speaker/video/data wires and only cross them at 90 degree angles (if possible). This will help minimize interference, which may create noise.
Finally, my most important futureproof recommendation. I'm installing a smoke detector behind the stage wall which will not only monitor the HT, but will also alert if any other smoke detector in the house is activated. If I acheive a 60 STC a smoke alarm outside the HT will not be heard over the audio track. Despite spending the better part of a year working on this HT, I would want to die there (at least not in a fire).
Good luck