I'm just curious (read subject). I have a hard time thinking of what I want to paint/draw. I work in a creative job (web design) so i've thought that this could be part of the problem.. hard to be creative 24/7.
Everywhere, anywhere and anytime. If all else fails, I just do some totally non related scribbling sketch and something usually gives me an idea even if it's just the way two lines look in relation to other, wierd ideas just pop into my head. If that fails, I look at other peoples art to inspire me to greater heights which usually allows me to be creative pretty quick.