A direct hit on the head!!
What is your weakness? This is a negative question which has to be answered positively.I know many of experts might have given you tips to answer this question. Look into the question first, its framed to convey the weakness in you.
To answer this question you have to analyze yourself before giving out the answer.
Every word you utter from your mouth matters a lot for this question.
I will give you my ideas to tackle this question taking myself as an example.
Even though I work as a HR in an IT company, before entering into the company I have taken 6 interviews. This question was posted to me in two interviews. So you can expect this question in all the interview as a Fresher.
 When they asked me, ” What is your weakness? I replied them with the idea that I got by reading articles on the blogs and websites.

The answer I gave out is ” I am short tempered, I want the work to be completed in the time and I expect the same from other people, I get involved totally with the job I don’t take care about my family. The HR said this sound too good but this is the usual reply I get from candidates. This in turn reflected in my selection. I was not shortlisted in the company.
I started to find out the answer for this question, after a week of hard work I found the answer lies in my resume. I had a consistence increase in my marks from my schooling to post graduation. When I was studying 10th Standard I scored 74% and it went on increasing and ended up in 89% while I completed my graduation. It was a continuous improvement.
In the next interview when they questioned me I replied them ” I am poor in studying, so I found that weakness and started to work on it. It has been reflected in my resume, where in my 10th standard I have scored only 74% and with the continuous effort I an able to reach 89% . Now I have corrected this one. When ever I find a weakness in me, I take this as an opportunity to make it as my strength.”
As the expert says, You need to convey a weakness which you had overcome and also quote it with the a valid example. Don’t say you don’t a weakness at all. Every human being on earth have a weakness.
You need to say a weakness which will not affect your job.
For example, when you are attending a interview to work in Voice process you are not supposed to say your weakness as not mingling with people, not talkative, have to improve my communication etc.
To quote another example, when you attend a marketing interview you cannot say ” I don’t have many friends, I am patience, I am not so aggressive etc.
1) Give out a bright smile when this question is asked.
2) Don’t shy out when this question asked.
3) Stay positive in all the words that you speak.
4) Be confident with the answer that you have given out.
5) Always quote an example.
Answer this question with a weakness that you have already overcome and validate it with an example. 

All the best for your interview!! 
Facebook ID: Balachandar Kaliappan
Twitter ID: uniquebala
Gmail ID: balasiddu@gmail.com