Technically I am not done with the theater yet. There is still some fixes we have to do with the door (shaving another 1/8", and painting) and I want to do some treating in the equipment room because there is a lot of echo in there right now.
So you asked for a once over. Here are a few things I remember that made a big difference.
1) I think if I did the whole room again I would have used blown-in cellulose insulation for my ceiling space. It really fills the joist gaps much better than roll-type insulation. Though the room is still pretty much dead quiet when sealed. Having done it twice it's just my preference.
2) I'd triple check if all of the walls were level. We had some minor variances on the walls being level that made a lot of our pre-fab column parts VERY snug. So this is a combination of checking level and just measuring each column rather than making them all the same.
3) Know exactly where you want your door before you put it in
Though I will be the first to admit, this really wasn't a lot of work to move.
4) Test any in-wall cables before putting them in the wall. My HDMI cable to my projector just didn't work at all. Lucky for us, it was in a conduit and the replacement worked.
5) Have a dedicated workspace outside of the theater room to do all of your construction. Working inside of the theater was a mess and there was tons of dust and cleanup we didn't need to have in there. If you have a garage, use it!
6) If you can get lumber/materials a little bit cheaper from some random place, stop to think about it first. Places like Home Depot will cut all of your wood for you for free. You might be able to save a buck somewhere else, but if they don't do free ripping/etc you lose a lot of time. Free labor is always worth a LOT.
7) This isn't something that hurt us on our build, but an FYI, order anything that is "Custom" in advance. Things like theater seats, custom hinges or doorknobs or linear grilles, etc. These things may take weeks to arrive and if you wait until you need them, your project may hit a brick wall.
8) HUGE one for us... PAINT COLOR! If you use paint, be POSITIVE you are using the right color before you do any work. I think I ended up painting all of the wood in my room 6 total times. I changed paint after the first coat of the light tray, painted everything else with 3 coats, found out the color looked completely different under the right lighting with the fabric in the right place, and had to re-paint again 2 more coats. Fabric colors do NOT look the same under direct light and they do not look the same if the angle of the light changes. Hung on the wall, the color was much different than laying on a table under the light.
9) Tools. For any power tool other than a drill (And sometimes EVEN a drill if you are using large drill bits) use CORDED tools. Battery powered things are great for small cuts, but for anything large you will need the torque of a corded tool. Ran into this with some cases of drilling and with a circle saw. Don't buy a tool you will only use one day. Rent it. Why on earth spend the money on something you might never use again. If you have help, you don't need a drywall lift. Just a "T" brace worked great, but we did have more than one person working to help. Two tools you absolutely MUST own. 1) A mitre saw 2) a quality cordless driver/drill. Depending on what you do with your own theater, you might need a good table saw. And you WILL need lights & extension cords.
10) Consider your treatments during your initial design phase. My treatment plan had 4" deep treatments on my rear wall. 4" is pretty thick. If you want things to be uniform, you should stop to consider what space your treatments will take up.
11) Help. Any helper, skilled or unskilled, is a huge help. A lot of times BIG and I worked together, one of us was holding something while the other person did the screwing/drilling. Large pieces, like 2' x 8' pieces of plywood, are heavy. Holding it up will be a major PiTA if you have to do it AND secure it. Think about things like lifting soffit pieces, or holding wall panels in place. Covering the fabric on the frames even.... an extra set of hands makes a HUGE difference in how long something takes and how well it gets done.
To be honest, not a lot of things were really lessons-learned, they were more just mistakes we made. The cable thing could have easily been prevented, but BIG and I checking each other's work as we did it solved almost all of our huge problems. So having someone else check your work is a big bonus!