I'll bite. Here are some of mine.
Buy more cable than you think you need. Those bends and turns add up quickly.
Don't buy all of the wall plates until just before you are ready to install them. You never know when your final configuration is going to change and you end up with odd plates/jacks.
Never assume that anything is squarely built in your theater, whether before you started, or after.
If you have an entirely enclosed room, don't forget the HVAC coming into the room, but just as important, don't forget some way for the air to get out of the room. Enclosed spaces warm up quickly even if you try to cram more A/C in if the hot air has no place to go.
Use flexible ductwork and several bends to prevent sounds from traveling through the entire house.
Soundproofing techniques don't have to be complicated, have many different options, and just require the correct application techniques to be successful.
Any room can benefit from acoustical treatments.
Use a good carpet and thick pad for your floor covering if at all possible. You might be surprised how many people sit on the floor.
Silicon caulk seals up seams wonderfully for air-tightness (yes I made that word up), but nothing wants to stick (paint, mud, primer, etc) to silicon caulk.
An extra set of hands can really be helpful sometimes.
An extra set of hands can really slow you down sometimes.
Take lots of pictures, not only to post online, but so that you can document what you did, where the wires run, and so forth.
No matter how hard you try, sometimes your camera will just screw up the colors and thus the look/feel of the room.
2-M60s, VP180, VP150, 4-QS8s, 4-M3s, SVS 20-39PCi, DIY Sub, 8-Shakers, JVC RS45