Lack of motivation is common factor in many a unpainted or unassembled army, with many gamers unable to bring themselves to devote the time to getting it done
Personally, i have mild Aspergers Syndrome, so once i'm working on something, i'm able to focus on it for hours at a time , only emerging for food and coffee.
However, finding the motivation to get into that 'painting zone' is a huge problem for me.
Luckily, there are two simple tricks i've found that work better than most:
1. Special Events
I find preparing for leagues, campaigns and special games like apocalypse, etc to be the biggest motivator, as I really don't like playing with unpainted forces.
For example, i'm in the process of painting a Dark Elf army that would never have gotten past the grey plastic stage if it wasn't for the fact that my local Gaming Club is starting a WHFB league in a few weeks time.
Like-wise, my Ork pirate army was mostly unpainted and sitting on the shelf until a 40k campaign started up in january. Now, after six months hard campaigning, which unfortunatly cumulated in the defeat of Waargh!!! Igork, I have 4000pts of painted Orks, including a Stompa to show for it, so i don't mind too much that i lost.
I find that in campaigns, because lists evolve as you go along, if you want to include new unit in your force the next week then you should say to yourself that you won't field them unless they're painted. This means that you're far more likely to get it done, as it's beneficial to your gaming, as well as the overall look of your army, rather than 'something i should really do, but can't be bothered'
2. 'What do i need to do?'
This is a technique that many gamers, including GW advise you avoid like Nurgles Rot, but it honestly does work.
Work out everything you want to get built and painted over the next few weeks and lay them out on your hobby desk. Then say to yourself 'I'm not going to paint anything other than what's here'. Then whenever you sit down to paint, whatever you decide to work on, it's progress in some form or another. This is what i'm doing for the huge backlog of stuff i posted up a week or two ago, and i'm halfway done already!