I cinch nearly all of my dresses at the waist with a belt, sometime to prevent the illusion of a circus tent that can be caused by a voluminous skirt. I find that pairing a shirtdress with a classic leather belt (preferably a shade on the brown spectrum) prompts me to envision past POLO Ralph Lauren collections. Wearing a polo shirt under the dress to obtain a double collar is also very POLO appropriate.
For a more contemporary juxtaposition, a skirt may be worn over the shirtdress and may be adjusted to either hide or conceal the bottom hem of the dress, according to the wearer's preference. I do recommend that when layering a skirt upon the bottom half of a shirtdress that the wearer opt for a dress of shorter length so as not to be weighed down by yards of skirt.