From Swim Smooth:
"... Correcting your technique is not actually that difficult, but you do need to know what to look out for and, just as importantly, work diligently to improve in these areas. If available to you, video analysis is a great tool for this because it really helps you identify what you personally need to work on. We recommend you get some if you possibly can. ..."
"...Swimmer's shoulder can be prevented by using proper freestyle stroke. The hand should enter the water with the small finger first and the palm facing inward. When the hand enters the water it should not cross the middle of the body to avoid impingement. For further stroke instruction, seek the advice of a swimming coach.
Swimmers should avoid rapid increases in training distances or frequency of training as this is likely to wear out the shoulder muscles leaving them at risk for impingement and shoulder pain. ..."