I really like this "elevator test" blog post from coding horror so I decided to make it a cheat sheet, you can get it as a pdf.
Basically you should ask yourself those 5 questions to keep yourself focus one the main goal: "solve the customer problem"
What are you working on?
Why are you working on that?
Why is it on the bug list?
Why was it reported as a bug?
Why does the tester think that?