Most people give a thiny veiled attempt at wrapping a strength up as a supposed weakness- eg I sometimes work a little too hard....
Don't speak about your vices and things you are struggling to overcome in your private life- uuuuh, well, uh, I tend to struggle with pornographic images on the internets and I have anger issues. That's none of their business.
Also don't speak about things that you can overcome with a bit of effort- ie I tend to day dream a lot and procrastinate.
When they ask what your weaknesses are, they are trying to understand what you are more suited to in your line of work. This will help keep you out of jobs that are not for you. If you get flustered talking to customers, you should tell them, because if you end up in telephone sales, you're going to be unsatisfied in your job.
So don't say it in an apologetic way- know who you are and how you function, and help paint a detailed picture.
So I would say something like I tend to be more creative working by myself than in a group. I'm able to see things in a way that other people often do not, but throw me into a room with ten other people in a group think session and I tend to fade into the background, and allow the more dominant personalities to push their ideas, even if I know mine are better. Lock me in a room for half an hour with a pen and paper though and I can come up with ten ideas that are usually very well received.