There are several methods of demand planning that businesses can use to forecast future customer demand. Some of the most common methods include:

1. Statistical forecasting: This method uses statistical models to forecast future demand based on historical sales data. These models may include techniques such as time series analysis, regression analysis, or artificial intelligence/machine learning. Statistical forecasting can be useful when there is a large volume of historical data available and when the demand patterns are relatively stable.
2. Collaborative planning: This method involves working with customers, suppliers, and other partners to gather information about demand and market trends. Collaborative planning can be especially useful in industries where demand is highly variable or difficult to predict.
3. Market research: This method involves gathering information about customer preferences, behavior, and purchasing habits through surveys, focus groups, and other research methods. Market research can be useful in industries where customer demand is highly influenced by changing trends or preferences.
4. Expert opinion: This method involves soliciting the opinions of industry experts, sales representatives, and other knowledgeable individuals to forecast future demand. Expert opinion can be useful when there is limited historical data available or when demand patterns are highly influenced by external factors such as the economy or regulatory changes.
5. Simulation: This method involves creating a computer simulation that models the behavior of customers, suppliers, and other market factors to forecast future demand. Simulation can be useful when there are many variables that can influence demand and when it is difficult to predict the outcome of these variables.

Businesses may use a combination of these methods to forecast future demand, depending on the industry, the product, and the available data. The goal of demand planning is to accurately forecast future demand in order to optimize production, reduce costs, and ensure customer satisfaction.