I read a blog of someone preparing for a 24-hour swim around one of the Swiss lakes and he was explaining about how it's important to get the reach and catch right. I understood this to mean getting your body rotation, shoulder and slightly-bent, extended arm into the most mechanically advantageous position. I imagine that this minimises "difficult" stresses on the shoulder joint. I'm trying to implement this without a trainer and it seems not to be irritating my (unrelated) shoulder injury.