A great swim tip which I have found very useful over the years, is to imagine that you have got a piece of string tied around your middle finger and someone is pulling it from the other end of the pool during the recovery part of the stroke.
This will do several things to your stroke:
1) It will prevent you from either crossing over the ‘mid-line’ during the hand entry or from entering the water too wide. This is VITAL to get your hand in a good position for the ‘catch’ – which is the key part of the stroke
2) It will prevent your thumb entering the water first and make you lead with your middle finger. This is very important to prevent shoulder injuries and to stop you hand gliding across your body under water
3) It will help to lengthen your stroke, preventing you entering the water too early and missing the front end of your catch