There isn't much I wish I had done different but here goes.
1. Spent real money to put in a better heat pump system for the room (this is 24x24x10 family room and I just cannot get the place warm during winter, summertime is OK).
2. Added some additional audio cabling (not that there isn't a ton in there now) up front since I am possibly changing from an unpowered sub to a sub with a built in amp in the front of the room. With that I wish that I had run some cabling for the possibilty to a sub inthe back of the room.
3. HAD SOME FRIGGIN' HELP!! This was the worst part. I basically did the project on my own and dam near killed me.
4. Need more space for DVD storage. Hey I thought I had enough but the collection is over 400 DVDs and there just isn't anymore room

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Things I did do right (most have been mentioned here)
1. Spent the money on the proper tools to do it right. If you can't do this than at least rent the tools you need to do it right --- THIS IS SO IMPORTANT!!
2. Ran TONS of cabling in the walls (including extra) even though I need more now -- ALWAYS RUN MORE THAN YOU THINK YOU NEED!!
3. Built a closet to house all the equipment!
4. Did 2 layers of drywall (that is some work!)
5. Did the work myself!! While this part does suck, I cannot tell you the incredible sense of pride I get everytime I go into that room and look around at what I managed to pull off. In addition I cannot tell you the smile you get when other people come over and marvel at your work

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As my wife always says .... "The whole process was one of the hardest six months ever but in the end it was all worth it because of the look on our daughter's face for her 3rd birthday party (movie party) .... You did good hun!"
What more can I say!
Best of Luck to all!!
Greg