Toddsterpatriot is correct that real wages have NOT dropped every year since 1973 or that real wages have NOT dropped 31% since 1973. However, the following graph may offer some clue as to what your source was referring to:
The actual numbers and sources are at http://home.att.net/~rdavis2/jobs.html. The data goes up to August of 2004. According to the BLS website, the latest hourly figure is 8.10 for October of 2005. In any case, the graph shows that the average hourly real wage did go down pretty steeply from 1973 to 1994 (though it didn't go down every year in that span). From January of 1973 to August of 1994, the average real wage dropped from $9.08 to $7.50, a drop of 17.4 percent. However, a quick google search showed where the 31 percent figure might possibly have come from. The following excerpts are from an American Prospect article at http://www.prospect.org/print/V7/27/sum-a.html:
From 1973 to 1994 the real median weekly earnings of young women fell 14 percent — compared to a decline of 31 percent for young men.
:
The earnings of full-time employed males older than 24 have also declined. The relative size of older men's earnings declines, however, has been much smaller—only a 9 percent decline for men over 25 compared to 31 percent for men aged 16 to 24.
Rather than real AVERAGE HOURLY earnings from 1973 to present, this 31 percent refers to real MEDIAN WEEKLY earnings for men 16 to 24 from 1973 to 1994. In either case, however, we are still well below the 1973 level.