There are two high seasons in the DR:
mid December, January, February, March, mid April
mid June, July, August
Thus, theoretically the best rates in the year would be available from mid April to mid June, and from September through mid December.
Given the 11 September events, and the November crash of AA587 flight, in addition to a recession in Germany, travel to the DR has declined this year compared to the previous year. Thus, you should be able to find very attractive rates even during the peak months. Shop around.