Note that this recipe calls for celery seed, not celery salt; if only celery salt is available, use the same amount but omit the addition of salt in the dressing. When testing the potatoes for doneness, simply taste a piece; do not overcook the potatoes or they will become mealy and will break apart. The potatoes must be just warm, or even fully cooled, when you add the dressing. If you find the potato salad a little dry for your liking, add up to 2 tablespoons more mayonnaise.