I need to forecast certain volumes months ahead at a time using Excel.

I know that recent campaigns have increased volumes, and I know roughly
when campaigns have begun, as well as the size of customer bases etc.

What I need help with is a way to decompose the sum of the weekly data
into roughly seperate 'signals' - once seperated, I will be able to use
the historic data for future forecasts.

Any ideas/help would be greatly appreciated.