Decide what type of self-publishing you are interested in. There are basically two types: offset printing and publish on demand (POD). Self-publishers who use offset printing produce a large quantity of books at once (like traditional publishers would) thus reducing the overall costs per book but requiring a substantial investment upfront. POD publishers have presses set up so books are printed as they are ordered. This means you don't need to store large quantities of a book you are not sure will sell, but you do end up paying more per book.
Become a member of Absolute Write, a free forum for professional and amateur writers (see Resources). The very active forum contains a series of sections dedicated to self-publishing as well as an area known as Bewares and Background Check. Here, writers post questions and experiences regarding publishers, and it's possible to find listings for almost any U.S. self-publisher (and some international companies as well).
Search the database at Preditors and Editors (P&E; see Resources). The number one database of writing services and publishing companies, the people at P&E specialize in bringing to light scammers, but they also review good publishers in standard print, self-publishing and electronic. Here, writers can find details on company policies as well as details as to what services are offered and what fees the author should expect to pay. Updates are ongoing so self-publishers are always rated based on the last few months of service.
Become a regular at self-publishing forums and sites. This will allow you to read what others are doing and to learn from their experiences. It will also let you see how well companies are dealing with their clients (the authors) and analyze complaints and praise from others. Examples of well-established forums are Absolute Write, Lulu (this self-publisher has its own forums that anybody can access), and the Writer Magazine forums (see Resources).