bass traps should be at least 4" thick, if your using something like owens corning rigid fiberglass 703 or similar product. Bass traps, normally built to be 2ft x 4ft in size should straddle the corners, this would include corners where walls/ceilings meet, but most people start with the main 4 corners of a room. FRZ (first reflection point) treatments normally are 2" thick, and as Royce stated should be placed in the reflection locations on the side walls and ceiling locations for your mains and center speakers. I had my son walk around the room with a small mirror while i was sitting in the primary listening location. Wherever you see the reflection of the tweeter is where a panel should be placed, centered vertically on the wall. In theory, you can't have enough bass traps, but you need to be reasonable.
An alternative to straddling bass traps in corners, is to use superchunks, they absorb much lower.
http://www.axiomaudio.com/boards/showflat.php?Cat=0&Number=144818&an=0&page=0#Post144818
