Most cost effective room treatment?

What type of improvements can be attained with acoustic room treatments. What treatments seem most cost effective?
You must damp all first reflections, even the AES standard room assumes that these are damped. I enjoy ASC traps very much, and they're as "cost effective" as most anything in this hobby, if indeed anything is so. I also augment secondary and complex reflection points with a few RPG profoam pieces, and two Echobusters panels.
Try carpet and curtains first. The improvements are not subtle. I recently changed from a light weight curtain to an extra light weight curtain. I immediately had bad reflections all over the 12 X 20 room. It sounded like a room before you move in the furniture. Before putting out extra cash make sure the basics are in order. Good luck.
1) Buy a house with a bigger room ! There is truly no substitute for a large room with high ceilings and heavy non-vibrating construction. I used to have one, and I miss it. 2) If you have a television, or even your audio racks between the speakers, you might want to try and move them to the side. Get them into an acoustically quiet area where the sound from the speakers won't shake up the equipment as much. I found huge improvements when I moved all my TV stuff out of my audio room. 3) Get rid of your coffee table, or put some sort of damping material over it. I had a large one with a reflective top. I tried removing it and found that I had better sound. 4) Tie your power cables together and route them down one side of your rack. Space your interconnects and run them down the other side. 5) Offset speakers at an angle to the wall so they don't have a flat surface to creat a standing wave against. 6) If you have a reflective wall next to the speakers, put something non-reflective at the point where the sound will reflect.
Try the site listed below for DIY treatments that work EXTREMELY effectively. These cost pennies on the dollar compared to most commercial pieces. If your handy with some very basic tools, you can literally save hundreds upon hundreds of dollars and build pieces that are "custom" sized for your specific installation and room. If your not DIY friendly, I would highly NOT recommend the RPG ProFoam pieces. These are the biggest rip-off / highest profit margin in audio that i have EVER seen. Sean >
Why don't you purchase a digital Tact RCS 2.0 roomcorrection unit??? In the end you will find that this is the cheapest perfect solution...
I made 4'x 2' panels consisting of a masonite backing board, framed with 2"x 1" poplar (like a picture frame, but assembled for depth, 3" would be better). Next take some spray adhesive (3M brand works well) and some R-12 insulation. Spray the inside face of the masonite, and stick the R-12 paper side down into the frame. Now, go to "Jo-Ann" fabrics and pick your favorite cloth. They have one that looks like old KLH or Advent speaker grill cloth. Stretch this over the face of the frame and staple or nail it around the backside. Now, hold this baby up infront of your face at arms length, and yell at it. You'll here some low midrange response (bass), but absolutely no high mid, sounds like you're talking through a pillow. Make a few of these, any size you want, and hang them behind you and to the side walls to dampen high freequency reflections. They really work quite well in my listening room at home.