Listening to TED talks or other motivational videos - most topics will do as long as it is somewhere around my area of interest and the speaker is good.
Also, changing of the work itself - I am tired a lot of doing one thing for a long time, so if I cannot force myself to make line art anymore, better start colouring something, to return to line later.
Drawing something else! Like, completely different, if you can't looking at those faces in the comic anymore, draw something "for yourself", just a sketch or two will do.
Definitely a lot of little deadlines. If you see that you achieved this little goal - you feel much better, than just diving into a vast ocean of everything at once and never finding a finish line.