For your strengths - tell them what they are - examples being responsible, energetic, always finding solutions to difficult problems. You should cite an example of an accomplishment of yours to support.
For your weaknesses - name one that is not central to the requirements of the job, say that you recognize that it's not your strong point, and comment that you are doing something to correct that weakness of yours. For example, if you are an accountant, you can say that you don't have a lot of technical knowledge about a certain software package, but did something about it by taking a class on it, quizzing your IT colleagues on how to work it, so that you are now pretty proficient at it.
Or you can say that you are a perfectionist - because you are detail oriented and responsible, but took too much time on minute things at the expense of the big picture. Now you learned how to make better use of your time by prioritizing and delegating some tasks to others.
Was in a job search group, coached people on job interview skills and tips.