Graph, thousand words, etc:
The green bars are the ones to pay attention to, representing how much the minimum wage in any given year was worth in today's dollars. What's harder to see is that this graph doesn't show you every single year. For instance, the really big drop about a quarter of the way in takes you from 1981 to 1988—the Reagan years, in other words. But while there are bumps here and there when Congress raised the minimum wage, overall we simply see that the minimum wage has not kept pace with inflation. These have also been decades in which minimum wage workers have become older, more educated, more likely to be parents.
This graph highlights why the minimum wage needs to be raised now, and why it needs to be indexed to inflation, stopping the pattern of eroding wages and then an insufficient bump up and then more erosion.