How to prevent shoulder injuries with doing a backstroke?

Form & function: Here are some thoughts: use proper form and prepare with good conditioning including calisthenics and stretching exercises. Stop or slow down exercising if you begin to experience pain. Gradually build up the duration and intensity of exercise. Get enough rest & recovery time between workouts. Get to know a good physical therapist.

Key is Form: You need to have appropriate form and endurance to maintain form. Additionally, stretching and core strength contribute greatly to maintenance of shoulder stability in the swimmer.