Most retailers, many wholesalers, and some manufactures promote certain sku's and in doing so spin demand away from past trends and absolute values. History is the planner's best friend. When the value of historical data is reduced, the planner is faced with a difficult challenge.
It is necessary that a formal process, such as Sales and Operations Planning (S&OP) be established to allow marketeers to articulate their expectations of demand. This demand forecast becomes a shared responsibility, as the planner will execute procurement decisions based on these promotional sales forecasts.
Further, it is necessary that the forecasts be "rolled up" and a sanity check versus budget be performed.
It can be done!