The way that you structure the problem will affect the forecasting method you need.Look first at the chart describing components of the sales forecast.
from J. Scott Armstrong (2001), "Introduction," in Principles of Forecasting, Norwell, MA: Kluwer Academic Publishers, p 5, reprinted courtesy of Kluwer Academic Publishers
Then consider the table below, describing possible methods for forecasting each component. The listed methods are likely to be most useful for the problem area stated. Particular attention should be given to the methods indicated in italics. Remember that combined forecasts typically lead to further improvements, so it is good to select more than one method.
For a further description of each area and the related research, see "Forecasting for Marketing" by Armstrong and Brodie.