As the above posters suggest, just blanket handing out CVs probably isn't going to be very effective. Employers can be very choosy at the moment and they are always going to choose the person that seems to fit the role best. That means for your application to be competitive, you are going to have to tailor it. You could argue that many high street retail jobs are very similar, which is true, but the candidate that expresses a specific interest in fashion, or books, or pets, or music etc is always going to fit Next, Waterstones, PetsR'Us or HMV better than the applicant that puts in a general application.
You should always apply with a covering letter, it's an extra chance to make you application stand out by saying why you want to work in that shop/organisation etc.