Rinse it with fresh water immediately.

Yes, that’s it. The salt is the problem, not the sand. Salt crystals dry into abrasive little knives that wreck elastic and spandex. A quick freshwater rinse after each swim stops that. No soap needed unless it’s actually stained.

If you can’t rinse right away, at least hang it wet in the shade and rinse later. Hot water, bleach, and washing machines are the enemy. Hand wash with a drop of mild detergent if you must, but cold water alone does the job for 90% of beach days.

Never wring it out. Roll it in a towel and press. Hang to dry out of direct sun. That’s it. Your suit lasts twice as long.