I moved about a year ago. In my old house, we had 1 big open room for the theater and entertainment area. My new house is a bit bigger and we're designing a dedicated 20'x16' room. I learned a lot from my last theater:
1. Estimate the wire length as others have said, then add a fudge factor (5-10% is what I'm doing), particulalry for pre-made cables. I underestimated my subwoofer cables and when my rack pulled out and turned, the cable pulled out and was hard to get back in.
2. If you're going to pre-wire and don't know where to put your subs, use wall plates with RCA jacks (I found them at Home Depot) in alternative locations. I'm now using HSU 1220s, which are passive subs with a seperate amp. They use speaker wire. This time, I'm pre-wiring for both the passive subs and future self-powered subs. Rather than having to fish for wire in the wall, I'm using wall jacks for almost everything. Audioholics may frown on this, but that's okay.
3. Projector outlet. I put it in the ceiling and covered it with drywall with no access panel. Not very bright.
4. I put the rack in the front - although it would have been hard to figure out in a big open room where else to put it, but I ad to turn on a light to see wha I was doing, and if in the middle of a movie, it disturbed everyone.
5. U shaped sectionals do not make good home theater seats. Go with either straight couches or something like Berklines. My wife and I compromised on the sectional last time, and the people sitting on the side had to turn their head to see the screen, making it really a 3-4 seat theater.
6. Make the rack bigger then you need. I added one shelf more than I needed last time, but them decided to add a video game, and wanted to add a place for records and a turn table (to go retro), the sub amp and other stuff. In you'e into this hobby, you'll find something to buy and need more space.
7. To NOT built in shelves to the sides of the front wall. This was another compromise. We put in display shelves for things like nice vases. Another mistake! I could never get them to stop vibrating.
8. To add an ethernet and phone line connection to the rack. I didn't have one last time, and now I will - and I'll make it accessible. I may even have 2 ethernet cables run for a PS3 and another component.
9. To design the rack in a way that allows a light to be added so you can turn it (and only it) on when you need to see in there in the dark.
10. Darker carpet. I used beige.
11. A black screen wall.
12. A shelf mount instead of a ceiling mount for the pj for a cleaner look.
13. Simple lights with an IR or RF remote that I can program into my universal simply. My lights were simple last time, but all manual.
14. A rack power supply/surge protector. I never had a power issue, noise from power, spike or anything of the sort. But when I wanted to access cords, I had to put my hand into the back of the rack and fish around. It was a pain.
I think that's about it. It seems worse than it was. It still blew me, my wife, kids and friends away every time, but a few nits and it could be been even better.
I hope that helps someone.