...small chunks - chunks so small that you can't help but easily complete them.... do the shoulder of one Fire Warrior your base colour tomorrow. Then his other shoulder the next day, then maybe his arm the next and so on.....
I agree that small chunks gets you there; however, I do not break up my time quite so much.
If I am feeling unmotivated I will decide to paint for 30 minutes then stop and make coffee whether or not I am in the middle of something. I often find that once I have made the coffee I return to my desk and finish off what I was doing then do some more.
...doing a log and setting some goals helps to get going and keep you focused.
I second that; having people tell me they want to see the next stage of a model makes the less interesting painting seem more worthwhile.
...a self reward system. I buy a model or kit that I have wanted for some time and then let it sit, unopened on my desk.
I have tried this; however I have not had much success as it makes me feel that there is something else (i.e. the new kit) that I could be doing instead, so it depends on your personality.
Something that has worked for me in the past is switching projects. When I lose motivation with painting I start something completely different, for example the conversion that has been lurking in the back of my head for weeks then, when the conversion is hits a bump I have the painting to do whilst mulling over options. I have not measured exactly, but I probably spend more time on the two projects together than I would on each individually so net gain. It also works for me if i keep two painting projects on large trays to I can swap which one is on my desk.
Ultimately, I find that I lose motivation when it starts to feel like a long grind or a chore, so avoid techniques that make me guilty about not painting; otherwise I find myself cleaning the bathroom to avoid painting.