Keep the sketchbook close (open!), even as pupils explore other areas. Encourage them to use it as a notebook, reference point, place to scribble ideas, whilst they are working in other media.
Sometimes during this stage, throwing in a left-field sketchbook activity can also help freshen or challenge thinking, so don’t be afraid to present the class with sketchbook tasks which appear unrelated to the main task in hand.
And remember where one project ends, another begins, and the sketchbook can be a vital tool in helping pupils understand both where they have been and where they might go.