(Also, see a graph of the changes in the divorce rate for the past 100 years.)
The best way to look at the divorce rates over time is to see how long marriages last by age cohorts (all the marriages that occur in a specific 5 year period). This figure is part of a 2011 report by the U S Census Bureau on timing and duration of marriages.
To understand this figure look at the percentage of marriages across cohorts that reach 5 years of marriage. Over 90% of those married between 1960-64 had marriages that lasted at least 5 years. This rate declined for most of the next cohorts and then by 1995-1999 the percentage making 5 years of marriage rose back to 90%. You can see the best evidence of the declining divorce rate by looking at the shift in the pattern of marriages lasting 10 and 15 years. Those marrying in the 1980s were more likely to have marriages lasting at least 10 and 15 years. As we follow these couples over the next 20-30 years they are likely to continue to have longer marriages.