Moisture and humidity can make your salt and sugar clump in their shakers. Frustrating, but easily corrected. I use this tip that my Mom passed on to me: place a saltine cracker in your sugar and place some rice in your salt.
They will absorb the extra moisture without changing the flavor of either and they won't clog your shakers. The rice can be used over and over, but the cracker might need replaced every couple of weeks or so. I had to use a club cracker to fit inside the opening of my sugar shaker.