There are many techniques used in forecasting. These are covered in the area of study of Operations Research (OR) or Business Operations Optimization. Two main stream categories are commonly in use, (a)Time Series Models, and (b) the Multiple Linear Regression models.
These techniques, are useful for mathematical prediction purposes, but should not be used blindly. A good understanding of why the past demand behaved the way it did, must be understood as well, and that projections into the future, should be tempered by the possible effect of future events or business conditions, relative to the products or service being analyzed. Remember, flexibility is always part of the key for a successful business.