Many days ago I found a site that shows more than 500 data models. This site is run by Mr. Barry Williams. I found these models helpful for starting database design. And I think that anyone can learn professional database design from these models. Hope you will find something useful from this site.
So, before start designing a database visit this site. Who knows, your problem might have already been solved.
Site address: http://www.databaseanswers.org/data_models/