The first is to organize the working materials for projects into their own, physical, folders. Then when you need to return to the project you just get the folder and you immediately have all the materials you need to hand. Substitute “area of the studio”, “rack”, “drawer” or “portfolio” to taste. You don’t need separate sketchbooks, and the system allows for an unsorted folder and other ways for ideas to mix and percolate.
The second is to break tasks down into quickly achievable “next actions”, ideally achievable in 20 minutes or less each. So rather than “make 20 paintings for the show”, “mark up the next canvas” or “sketch the cat”. Yes, much of the value of art is long periods of visual contemplation or creative “flow”. But you need to get there, and worrying about the big picture is less constructive than building up to that state slowly but surely.
There may be more in Getting Things Done of interest to individual artists, and its system is applicable to admin and other non-art work as well. It’s well worth a read.