Most guys only Bench, usually with bad form, and neglect to strengthen their upper-back with heavy Barbell Rows and Deadlifts. Add excess situps and you end up with a hunchback posture and sooner or later shoulder pain.
Aside from proper programming so this doesn’t happen in the first place (shameless plug: do StrongLifts 5×5), you should stretch your chest muscles. The video below shows one simple yet powerful exercise to do this – the 1-arm doorway pec stretch. Video by Eric Cressey MS CSCS, author of Show & Go.
Position your arm against the edge of the wall (or a doorframe). Now put your shoulder-blade back & down (imagine trying to pinch something between your shoulder-blades). Then use the edge to push your shoulder joint back and turn your head in the opposite direction to increase the stretch on your chest.
Hold the stretch for 15 seconds then do the other side. Better to do this post workout since research shows that static stretching pre-workout inhibits maximal strength. If you have very bad posture and experience shoulder pain when Squatting/Benching, do this stretch daily for faster mobility gains.