Have something prepared, you know why you want to work for them but why are you the best candidate for the vacancy and what are you going to bring to the position?
Work out what your strengths are and sell yourself based on these strengths. If you have any weaknesses don't reveal them as such but do discuss them in terms of things that you would like to learn in order to make you a more rounded and valuable employee.
Don't rock up on your bike in your lycra gimp suit - an employer wants to see who YOU are not who you think they want you to be or how you percieve an LBS employee to dress, and as you will be customer facing first impressions count. This harks back to an earlier thread about what's acceptable in terms of workplace attire and interview dress codes.