Relying too heavily on 401(k)s alone. Tax-protected retirement accounts, including 401(k)s and 403(b)s, should be just one of three parts to a retirement plan, says the Women's Institute for a Secure Retirement. The other two are personal savings and Social Security. It's that first one—personal savings—which often gets forgotten, partly because people find it difficult to save any money beyond their employee-sponsored programs. Relying exclusively on Social Security, as ING DIRECT found 1 in 5 Americans were doing, is just as problematic, since most people's Social Security checks alone don't provide for a comfortable retirement.
In a study of low-income women, Helen Levy, researcher at University of Michigan's Institute for Social Research, found that people want to save more, but they find it impossible after covering basic needs and the burden of supporting needy family members. As one respondent put it, "I'd be great at saving if I had any money."