An interview is a stage where you have to sell yourself hard. Considering that you are applying for a sales job, if you fail to sell yourself first, the job is not yours.
You should be able to answer the obvious job related questions. This section forms the crux of any sales interview. Interviewers will want to know how much you know about the sales job. Hence, prepare yourself well to be able to answer them with least difficulty.
Carefully choose your words while discussing questions related to handling the customers, resolving their problems, increasing the sales volume etc. Evaluations are based on the manner in which you tackle these issues.
If you are a sales professional, then it is expected that you look like one. Formal attire combined with a business like attitude defines a good salesman. A business suit or even a shirt and tie combination is widely accepted attire for a salesman.