One of the most popular interview questions asked of candidates in the job search has to be "Tell me about your strengths and weaknesses." Answering the first part of that question is somewhat easy for most people to answer, but very few people answer the second half of that question effectively. They either give the standard, generic answers (e.g. "I work too hard", "I’m a perfectionist", etc.) or they give a weakness that has nothing to do with their performance on the job. In my last position I asked someone to tell me something that they are working on from a professional standpoint (a.k.a. their weakness) and they answered simply with, "Gardening. My husband keeps telling me that I need to plant some flowers." Let’s just say that I was less than amused and ended the interview as quickly as possible. That response had nothing to do with her quest for a new position.
A more effective way to answer that question is to reflect back on your most recent position and create a list of skills that you were not able to further develop or improve. Keep it positive. Keep it relevant. Most importantly make sure you explain in a clear and concise manner as to how developing those skills can be an asset to your future employer. I also recommend giving a little bit of detail on how you were able to improve that certain aspect even the slightest bit in the last position. If you can explain yourself effectively, a potential employer is going to appreciate that you have recognized a genuine need for improvement in a certain area and that you have also already taken the time and energy to analyze how you could work on that area.
Just remember, practice makes perfect so solid preparation for this answer will drastically improve the eloquence of your response.