Practice rolling up a bandage and lead rope - take your hat incase they want to see you ride or handle horses (if it is being done by the book), and dont forget your gloves.
Apart from that, tell the truth about your experiance and dont say you can do a full muck of 6 stables in an hour - unless you have seen the state and size of them
Yes agree, don't overestimate ANY of your abilities but at the same time, don't make out your useless when your not.
I think you will get a pretty good idea of the 'kind of yard' it is when you arrive and you can go on from there to try and merge yourself into their way of doing things.
I think when it comes to working in yards, common sense and hard work go a long way. I wish you all the luck.xxx
Think of the questions you want to ask as well, e.g. what will be your regular duties, what hours will you be asked to work, which days will you get off, who will you work with, how long did their last groom work for them and why did he leave, etc. If there are any arrangement for you to keep a horse there or get lessons or training as part of the job make sure it is very clear what you will receive and when.