The starting point for your sales forecast is last year's sales. Before you factor in a new product launch, or an economic trend, look at the level of sales for each customer last year. Do you know of any customers who are going to buy more - or less - from you next year?
In the case of customers who account for a significant value of sales, you may want to ask them if they plan to change their purchase level in the foreseeable future.
Consider how many new customers do you gain and lose each year, the average level of sales you make to each customer, whether there are particular months where you gain or lose more customers than usual. How much does each customer buy?
Every year is different, so you need to consider any changing circumstances that could significantly affect your sales. These factors - known as the sales forecast assumptions - form the basis of your forecast.
Being over optimistic with your sales forecasting will not help your business. It's a good idea to look back at the previous year's forecast to see if your figures were realistic.
For specific examples and detail on forecasting visit the Gov.uk site.