First, you need to distinguish between bass(y) traps that are really for lower midrange to upper bass unless montrous and absorbent panels for slap echo and first reflection. For bass, the most effective places are the corners, any corners and bulk is necessary to get them to work low. For the others, free-standing or wall-mounted panels of 2+" thickness are useful, with or without some HF reflection.
If you read the info at www.realtraps.com, there is lots of useful information and the products are good. Another source of information is www.rivesaudio.com. I've used panels and materials from Auralex, ASC, EchoBusters and http://www.sensiblesoundsolutions.com/