A sound principle to follow in demand forecasting is to

One of the most important principle in this regard is the appropriate product history which effects the future demand of the product.

Explanation:

Demand forecasting is to make predictions and forecast the demand of the product that has been produced by a particular firm. Since the demand has to be forecasted and predicted, a lot o factors which might be controllable and might be uncontrollable factors have to be taken in mind to forecast the demand.

The things to be kept in mind in this regard are the history of the product where previous sales will affect the prediction of the future demand, the promotions of the product, the trends and so on.