Set a timer
X minutes working, 5 minutes off.
I noticed I tend to be most motivated with the ticking clock at a figure drawing session- gotta get it done before the end of the pose! So I decided to bring that home.
Been experimenting with various times for working- have been starting with 5 minutes on, then increasing by five minutes each time. Despite the somewhat inefficient use of time in the beginning, something about those first ones being short seems to get me going.