Put a project on a deadline and it will get the most attention. After that, it comes down to parts in hand, whether my wife or I specifically want to ride it (which is sort of like a deadline). Things that derail projects for me are yard work, stuck seatposts or bottom brackets, or trying to decide whether or not to keep it, and demotivation due to nobody buying what I've finished up. I am an interesting (or not interesting) mix of optimism, disorganization and procrastination that occasionally gets in gear.