Dont rush answers when they ask you something, think it over first. Otherwise you'll say 'Ummm' and 'errrm' alot, and look a bit stupid, especially if they've asked you a relatively simple question.
Always give answers in the most positive way possible, and never insult your present/past employers. Your strengths are your strengths, and your weaknesses are also your strengths, dress them up. You're not a slow worker, you're sometimes 'too much of a perfectionist' etc.
You can never be over dressed for an interview, but you can be under dressed. No one ever got negative feedback for wearing a tie, but you can be sure that the person who didnt was spoken about as 'not giving a professional vibe'.
Speaking of professionalism, if you add your email to your CV/Resume, make sure its a sensible email address like
your.name@gmail.com, and not
SeXXi-BABEEE218@hotmail.com. People look at that and immediately throw your application away.
Be 'confident', which of course isnt easy if you arent. But its easy enough to fake, just hold your head high, grip their handshake firm, make good eye contact and speak clearly.