Premature wearing of the middle of the tread is because of over inflation typically, but also could be from "cupping". Cupping occurs commonly with larger tires (bigger than stock) on rims that aren't the optimum width. Take me for example, my 35X12.50X15 BFG A/T KO's are mounted on 15X8" rims... the optimum width rim for these tires is 10" wide. So having 15X8" rims is causing the sidewalls to suck in on the rim, ballooning the tire outwards which causes the tires to look like a 1/4 moon (when viewing front to back on the tire, not from the sides) as opposed to being flat.
Wear on the edges of the tire is normally from low tire pressure (the tire is riding on the sidewalls instead of the tread itself). Back to my truck... in my case because of running 15X8" rims if I run the tires with 32psi in them they will cup, so for around town I run 28psi (helps a little) and when I go offroad I go down to 15-20psi. However on the highway I like to keep it at 32-35psi so the truck will be more responsive.
Hope that helped a little, let me know if it didn't and I'll try to reword it.
-Chris