Ever since I can remember my mom has put a few grains of rice in her salt shaker. And so, I have always put a few grains of rice in my salt shaker (~ 1/2 tsp.). The purpose of the rice is to absorb moisture and prevent the salt from clumping.
Since I have always put rice in my salt shaker, I don’t actually know for certain that my salt will clump without it. But since I live in a humid climate, especially in the summer, I figure it’s an easy enough tradition to continue.