If you do not have the design experience necessary to create a professional looking online store, and you don't have the funds to hire a professional designer, you might consider using a Miva Merchant Skin. For a relatively low price (discounted at the moment), you can have a professional-quality design that you can customize fairly easily. At the time of this post, there are 26 designs, from Children's Store to Electronics Store.
I have created four tutorials for the Advanced Subscribers about how to work with Skins. However, in order to help those who are thinking about purchasing a Skin or who have bought one and are struggling, I'm putting them here for everyone at no charge.