Plenty of good advice here already. All I'll say, speaking as a manager in the retail-sales sector, is confidence!
A sales assistant role is 10% knowledge, 90% people skills - product knowledge can be learnt, personality can't. Be confident without being arrogant, be talkative and engaging without waffling.
I've interviewed a fair few down the years, and mumbly bumbly people are generally an instant turn off (so to speak). You just need to show keen interest, that you can engage and converse comfortably and demonstrate a little bit of initiative. Also, gear all of your answers towards the customer; "whats your idea of a good shopping experience" is a typical question. Don't talk about yourself, answer along the lines of "its about listening to the customer, making them feel welcome and showing that you want to help them" or something slightly more eloquent. It demonstrates a general awareness of what customer service is.
Questions are a good one. I've forgotten to discuss things like pay and shifts before, and when I asked at the end of the interview if there was anything they wanted to know, a "no" was a good conclusion. Asking about the chances of development or progression shows commitment and a desire to succeed.
Another good question to throw at them right at the end of the interview is "Do you have any concerns about me as an applicant?". It can actually throw them off balance a bit, subtly reinforces the whole confidence-to-do-the-job thing, but most importantly you are inviting them to talk to you about the things that may be rattling around in their mind which may cost you the job, things which you can then attack again and hopefully clarify or correct for the better.
Good luck.