website was one of the first sites I started learning about things. Of course, they manufacture their own products, but Ethan has always promoted DIY, for folks that would like to build their own.Ethan's Acoustic Treatments and Design page
also has tons of info.
You can also just google "DIY Bass Traps" or similar search..
In a nutshell, low bass collects in corners, that includes floor/wall and ceiling/wall. However, most of us can't apply bass traps to every location.
The corners can either have panels stradling them (at least 4" thick), or superchunk triagles filling the void.
First reflection panels, are those that are placed on the wall/ceiling at the point where the first reflection comes from the speaker and bounces to your ears. This reduces echoing and the skewing of the sound arriving at your ears at different intervals. You don't want to completely deaden a room, but treatments are just as important in a HT environment as they are in recording studios.
I think Ethan's room has over 20 panels and he says it is about as acousticaly flat in frequency response as it can get. He can pretty much walk around the room and find very minimal peaks/nulls without multiple subwoofers or EQ. He is not a believer in EQ.
There are also companies that make very affordable great products, so you don't have to mess with making them and time required. RickF has purchased from GIK Acoustics
and they make nice stuff fairly a affordable...