How to keep macaroni salad moist?

America's Test Kitchen/Cook's Illustrated suggests adding a little warm water when your macaroni salad gets a little dry. Most of the drying is from the noodles soaking up liquid from the mayonnaise, so adding more mayo tends to just make the salad too oily and isn't really needed anyway since all you really need to do is rehydrate the mayo that's already there.

With pasta too dry i often add a splash of milk to moisten ... Doesn;t hurt flavor adds to liquid.

Apply plastic wrap directly to the salad, so its touching. Make sure to bring it all the way to the sides of the bowl. This prevents moisture from evaporated inside the dry environment of the refrigerator.