Warm up sets are very important.
As is realigning or popping out-of-place joints, especially in the neck and back, before heavy sets.
Yesterday, although I warmed up with five minutes of rowing, I did not remember to stretch-pop my upper back into proper alignment before doing lat pull downs. As a result, I am extremely tender in my upper back/spine.
A great site for exercises (and diagnosing pain) is www.exrx.net. Based on the information here, my pain appears to be right where the rhomboid attaches to the C7 vertebrae.
image source: http://www.exrx.net/Muscles/Rhomboids.html
