Two ideas: (1) Find a partner for each side project, or at least 1 target user (real person you know) you commit to serving by a hard date. It's an uphill battle to maintain accountability without another human. (2) I draw a box on my whiteboard and write in it the essence of the project and only the 2-3 deliverables essential to achieving it. When I take on a new activity, I challenge myself as to whether it goes in the box or represents a branch from it. Forces me to visually acknowledge how I am spending my time (and thus define and stay to the essence of the project).