This post is in response to Ron's collaborative post idea. Seeing as I teach high school, half of my job is keeping lazy kids motivated. So here's my thoughts on the subject.
With my students they can be motivated by many factors, the most pressing being grades. However, the best work that I receive is from students who are motivated to create a good project. I have students who will sometimes not move on to the next project because they are determined to make the best work they can on the previous project. Of course I generally let them; I think they learn more from completing a fine piece of work turned in late rather than turning in a half-finished project on time.
Anyway, with Warhammer your best work comes from being motivated. Whether your motivation is a sense of pride, money from a commission, or to outdo a rival, any of these goals will keep you working. If none of those apply to you, take motivation in having a completed army! That's generally motivation enough for most of my projects.
Every project of mine has a different motivation. For the Malanthrope, I'm working out of a sense of pride more than anything else. For the Deathwing, that was for money. With my Titan I was trying to outdo the other scratch builds I saw. For most everything else I have worked on, my motivation was having a finished product looking nice on the battlefield.
So here is my final advice: If you severely lack motivation, you need to experience (or re-experience) the fantastic feeling of seeing your own painted and based units on the field. Chain yourself to your painting table or whatever it takes to get a small unit or model painted. The next time you play, include that model/unit. You'll find yourself a little more motivated after seeing how nice things look finished and on the gaming table instead of the painting table.
Here are some general tips: Remove distractions. I tend to paint a color and then drift over to the TV and play some video games- next thing I know I've lost my free time and am behind. Although research suggests that background music is distracting for normal tasks, I feel that it actually enhances artistic projects. When I try to work in silence I am very easily distracted. Music helps me stay focused.
Don't look at your whole collection and think about what isn't painted. I suffer from "buy faster than you can paint" syndrome. As a youth, I had very little disposable income but lots of free time so when I bought something, I got it painted. Now I have more disposable income but much less free time, so my pile of unpainted models is growing faster than I can paint. Not to mention, I need to strip the paint off my old projects and get them up to my current standard. So instead of looking at the whole, look at the part. Pick a squad/model and focus on that to the exclusion of everything else. Take a little bit at a time and soon enough everything will be painted. This is working for me as I'm finally starting to catch up on my backlog.
Finally, I set "color" goals to get as much done as I can. When painting a squad, I do one color at a time. To set a color goal, I say to myself I can't get up from the chair until I have finished with the black. Once every model has everything needed picked out in black, I get up and get a glass of water and maybe a small bite, then set the next color goal, and so on.
So find yourself a goal or a reason to paint, and just do it. If you can't find a reason, make one yourself.