Just say what I've been saying ever since I listened to Mitch Hedberg's CD: "Celebrating the 5 year anniversary of you asking me this question!"
In all seriousness, it's really hard for me to answer this question too. I never would have guessed I'd be where I am now five years ago and I'm sure in 5 more years, I'll be surprised where I am too. I think it's tough for me because I don't adapt well to change, so I oftentimes see myself repeating the same routines over and over when I get asked this question. And I'm sure the person asking doesn't want to hear "same stuff I do everyday: work, game, write, listen to music and play guitar".
So to help myself answer that question, I just think of things I did as a kid: what do you want to be when you grow up? And things start to come to mind. Like: purchasing my first home, raising a family, working my way up the corporate ladder, being a football playing king in space, etc. I'm sure you have aspirations in life (everyone does). Just use those as your fuel to answer this question.