To clean away soap buildup, rub with a mixture of 2 tablespoons salt and 1/2 cup lemon juice.
Or an alternative that many experts swear by is a mixture of baking soda and water.
Use 1/2 cup of baking soda, then add water as needed to make a thick paste. Using a nonabrasive sponge, scrub the glass and rinse it with vinegar. Easy as that.