Depends on your particular setup - the best way is to plan for cabling before construction starts - floating floors are good places for cable trays - in residential construction, sometimes under the house and back up works well (if you're rural and ever have rodent problems from fields, etc, use conduit that's large enough for any "un-removable" computer cable ends, and stuff both ends full of fiberglas insulation after all cables are in.)
If you go in-wall, offset boxes by two stud cavities (leave a "virgin" cavity between the two that have a box in them) and caulk EVERYTHING inside the boxes, around them, total seal is necessary. Even well-sealed boxes, if they're back-to-back, will cost you anywhere from 6-15 dB of isolation. Boxes that are recessed into the wall (and properly sealed), without another box nearby, will only cost you maybe 2-3 dB due to replacing wall insulation with a box full of air.
Always put at least 2-4 more circuits of each type in than you think you'll need - sometimes, even DOUBLE isn't excessive in the long run. Once things are in place, you do NOT want to open this up again unless something breaks.
Hope that helped... Steve