Thing is, you have to say something, and that something should not be anything that will negatively impact your ability to perform the job. It is a trick question designed to weed out people who willingly give up exactly why they should NOT hire you.
So it needs to be either of the following:
- A weakness that does not relate to your performance of the job at all, but you also need to point out how you ARE working to improve it.
- A weakness that also has an upside (I work too hard is only the most obvious of this, it's probably not a good idea to use it)
For a joe job, you should just go with the second one. There are no competent hirers at joe jobs. Or at least not enough that it matters. For 'real jobs' you probably should use the first. In the end, though, it is harder to come up with one that fits the first than the second.
What it really comes down to is that the person looking at these is not looking to be your buddy. They want to know if you can do the job. The weakness question is a trap, and you need to be very careful in how you answer it.