Actually I found out, that the best killer for lack of motivation is...
No, seriously, eventually you'll be bored out of your ass that you'll go "Eh, guess I'll draw a bit." and before you know it, the artwork is complete!
But then again if it ain't artwork, there'll be other hobby you'll approach the same way, like... learning new language? Exercise?
Boredom gets to you, one way or another. The last thing a human wants, is lack of goals in his life.