Choose a weakness that is related to a skill and not a character trait. Character traits are difficult to change"”for instance, you may naturally be inclined to keep to yourself (e.g. introverted). Skills, on the other hand, are learned behaviors. Think about a skill that you can learn more about and ask yourself if it's a critical error that would cause a lot of trouble. If it is, skip it and move on to a less critical skill.
Talk about how you're improving or intend to improve that skill. If possible, show that you're internalizing this improvement so that it becomes a permanent change.