Aside from directly applicable undergraduate degrees like Occupational Health & Safety, Human Computer Interaction, and the elusive undergraduate Ergonomics program; some of the most common and viable disciplines you can study are:
Psychology
Engineering
Computer Science
Graphic Design (fine arts)
Kinesiology
Occupational Therapy
Sociology
Management/Administration
Any of these can provide you with a background and essential skills that will supplement your job in ergonomics or usability. Some universities even have departments in the above-listed disciplines that offer undergraduate programs with courses in ergonomics and human factors,
While nearly all professionals hold a bachelor's degree, approximately half have attained a master's degree. If you're willing to go the extra distance you could pursue a master's in one of the previously mentioned disciplines, or...
If you have a reasonable GPA and are willing to travel you can find masters degrees in ergonomics, usability and HCI at the following universities:
In Canada,
University of Toronto: Human Factors Engineering
Carleton University:
Human Computer Interaction
University of Calgary: Cognitive ErgonomicsUniversity of Waterloo: Ergonomics in Kinesiology
In Britain,
University of Derby: MSc Health Ergonomics
University College London:
MSc HCI/Ergonomics
Loughborough University: MSc ErgonomicsNottingham University: MSc Human Factors
Nottingham University: MSc Applied Ergonomics
Cranfield University: MSc Ergonomics & SafetyBrunel University: MSc Human Centered Design
In U.S.,
There are over 30 insitutes that offer Human Factors, HCI, & Ergonomics in the United States; see Usernomic's listing for more detail