The question "What are your biggest weaknesses?" is both one of the most common queries and one of the most poorly answered. Since you likely already are nervous, the last thing you may want to talk about is your shortcomings. If you carefully plan for this question before your interview, your answer can make a substantially positive difference in your interviewer's perception of you and can turn your weakness into a strength.
It is difficult to discern a prospective employee's work ethic from an application or a resume. While you can glean some information from a job reference, you cannot always tell how dedicated an employee is by someone's recommendation. Answer this weakness question by telling your interviewer that you sometimes have struggled with maintaining a good work-life balance. Mention that you have reflected on the concept and realize that not only an employee's service will suffer if this balance gets thrown off, his family life will too. This will show that you are more than willing to work hard, but care about your family and your performance as well. This also will allow you to avoid corny answers like "Sometimes I work too hard," while still showing your work ethic.
Work in Progress
Another trait that can be difficult to pull from a prospective employee is self-reflection and self-improvement. Every employer wants an employee to grow and most are thrilled when their employees take the initiative to grown on their own. Answer this question with a real weakness that you are working on and growing through. For example, if you struggle with time management, cite it as a weakness, then immediately and specifically tell how you are working on the deficiency. Explain that you are using technology to become more efficient or you are re-evaluating how you schedule projects or meetings to improve in this area.
Cite that you put too much pressure on yourself to succeed or that you have very high standards for yourself and your work. This shows that you want yourself and your employer to succeed and should require minimal motivation. This answer comes with a caveat that you may need to explain. Some employees with high personal standards and goals subconsciously or otherwise hold others to the same standards in a way that goes beyond their ability or desire. While it is good to hold a subordinate to a higher standard, and provide them with tools to grow into the standard, it can cause conflict to transfer your high standards to peers. Explain that you share tips, set a good example and work well as a teammate to improve the whole team for the betterment of the company in a way that doesn't disrupt peer relationships.
Public speaking is often cited as one of the biggest fears people have. Cite public speaking as your biggest weakness. This will allow yourself to appear humble and will be an easy weakness in which to demonstrate improvement. Discuss how you learned that the more you prepare and practice a speech, the less it becomes a weakness. Discuss ways that you have made inroads in the topic. Cite examples of how you have placed yourself into situations to speak in public so that you can get practice and improve. This demonstrates self-motivation and a desire to improve.