When interviewers ask you this, it's not to pin point your flaws. Rather, the purpose of this question is to see if you are self-conscious enough to address your flaws and to also know what you are CURRENTLY doing to improve on this weakness. How I address this question is mentioning how I can some times overwork myself by taking on too many responsibilities at one time and how it has lead to many sleepless nights. I have been working on that problem by keeping multiple forms of schedules and calendars to keep track of my responsibilities at all times and to allot a little more time per task than I presume I would need in case mistakes, errors, or tangents occur along the way.