just go to an empty server or map, fix you crosshairs on a stationary point at head level and try moving around, jumping crouching etc without making the crosshairs move too much from the point...It really helped me, I improved in one week and was fragging away
Also try to join a new clan when you get a little decent, clan wars will take you a long way..I improved immensly when I joined the oops clan in enemy territory, but after 2 months the clan fell apart...then I joined the $i clan for a few months and soon moved to cod4.