Identify the character traits that you want to describe. This will allow you to get to the "heart" of the matter. For example, a weakness might not be that you are "lazy"; it might instead be that you are "unfocused" and "easily distracted." Similarly, it is unlikely that a strength would be that you are "successful"; it might instead be that you are "driven" or "tenacious."
Show how these strengths and weaknesses have come into play. The most impactful descriptions are ones that rely on anecdotal evidence that demonstrates these traits, not just lists them for an audience. For example, instead of saying that someone is effective at time management, detail a time in which the individual had to accomplish a number of different tasks in a short amount of time.
Describe these traits from both positive and negative perspectives. This is especially important for a job interview. For example, if your weakness is that you can be unorganized, you might also add that you have recognized this flaw and you are working diligently to correct it by implementing personal organizational practices, such as structured note taking, list making and using computer devices to keep yourself on track.