Steve Jobs gave the commencement speech at Stanford this year and while his advise is hard to follow, it is inspirational. He says:
And the only way to do great work is to love what you do. If you haven’t found it yet, keep looking. Don’t settle.
He says every morning you should ask yourself if what you plan to do today is what you’d do if you only had a day left to live. If you answer "no" too many days in a row, you need to find something different to do.
Here’s the whole talk: ‘You’ve got to find what you love,’ Jobs says.