Decide. Decide which project you are going to finish. Spread out partially done work and take a careful look. One will likely send out a little magnet or tug on your coat. Pick a project, any project. Put the rest away. If you are starting fresh, choose a topic or structure to start with. In "The Cauldrons of de Kooning," the author quotes de Kooning, "In art one idea is as good as another." Ideas that have meaning to you, that is. If you like, you can order a mug with this saying.
Commit. Make it the top priority on the to-do list. Gather your materials, clear a space, and sit down.
Focus. Work on one section. Do you need to write? Edit? Design pages? Create images? Sketch it out, doodle on sticky notes, cover your table with brown paper and scribble. Focus on the one area, but let yourself make preliminary notes and possibly even mistakes on separate materials first.
The main point is that the project has to be able to re-grab us. Something inspired us to begin with and we have to recapture that mood, get in the groove again. It may or may not be comfortable going back there, but sometimes pushing through that discomfort is part of the process and the final outcome can be quite rewarding. We can also decide if we really want to finish or if we should commit to recycling the materials and focus on moving on.