Salt creep is often caused by microbubbles popping at the surface. If you can keep as much air as possible from entering the drain line, or allow it to go up instead of down, that's the first step. From there, the only thing to do is use a lid or something to help contain the little splashes that occur. I've done things like let filter media float on the surface, or pile rocks up above the surface so it catches most of the mist.