Think about the job and look at the job spec, if there is one.
Be honest about your strong points, and make them relevant to the job.
Employers like to hear about good organizational skills, communication and dedicatation. A willingness to train and advance yourself is also a good point to make.
That's the easy bit! The weakest points is the bug bear.
You have to make them positive to the company viewpoint.
Here are a few examples to think about:
'I know that I can get too involved with customer problems - I care too much. I have solved this by not getting personally involved and remaining professional at all times.'
'I tend to be too early for appointments - I hate keeping people waiting. But I am aware of it and am trying to manage my time better. I take paperwork with me, so that I can still be productive if I am early and have to wait'.
'I have a fear of failure and push myself just that bit extra to ensure I don't. I pay particular attention to detail and see a job through to the end.'
'I can get too involved in projects. But I have learnt that the work/homelife balance is important.'
Good luck!