I am so sad for you that you only have one day in Rome. I think Rick Steves is generally full of s***. Following his advice is a good way to look like an American and eat and stay only where the Americans stay. Sorry but its true. I have not a high opinion of his work.
My first piece of advice is do not wear heels in Rome unless you want to tear up the heel and break your ankle. I think a pair of chic flats or a stacked 1-2 inch heel (2 at most) should be sufficient.
Rome is full of fun places to shop, and where to go depends on what your taste and price range is. Rome is also one of the most enjoyable places to just wander around and discover gems hidden in alleyways.
The high-priced and luxury area to shop is around the Spanish Steps (Piazza d'Espagna). There you will find your common, luxury shops like Max Mara, Yves St. Laurent, la Perla, etc... Many of these shops are in and around Via Condotti, which starts at the Piazza. If you travel on either road extending from Via Condotti traveling towards Piazza del Popolo (Via del Corso and Via Babuino), you'll find other nice shops including my favorite Diesel store (for this one you'll have to turn left onto via del corso away from the direction of piazza del popolo towards Piazza Venezia), Armani Exchange, nice small boutiques and an Italian Brand that I love known as Ethic. There are several Ethic stores around Rome, and it is similar in style to Anthropologie. On Via Del Corso, walking towards Piazza Venezia, you'll pass many cute boutiques and some chain stores like Zara.
Another nice and popular place to shop is around the Campo de Fiori. In that area, you will find many small boutiques and some Italian chains (like Intimissimi, Ethic). These are fun shops, just because they are on narrow alleyways.
If you tend toward smaller and less chain type stores, I recommend wandering through Trastevere (pronounced trahsTAYvayray--not trahstayVAYray like many tourists pronounce it--meaning across the Tiber). This is the "indie" side of town and is populated by many young people. There are so many cute stores, vintage and smaller boutiques and great places to eat. You really shouldn't miss it.
If you want a true Italian shopping experience, get up bright and early and head to Testaccio (get off the Piramide metro stop and head towards Testaccio)--this is where I used to live. There is a great market open in the mornings with clothes sold open air. Testaccio's market is known for its inexpensive shoes.
Have Fun.... I miss Rome so much. I think hubby and I are heading back for a visit this fall.
