I drag elements in as I go... keeping them as a smart object as I resize things often. If I need to do something to it, I rasterize it. I also merge papers down to the shapes I'm clipping them to as I go. When I'm sure that I'm done, I go to the layers panel and filter only for smart objects. Then I select all the smart object layers and rasterize them all at once (I have the rasterize function on a short-cut) to save space!
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