My biggest tip is just to be yourself. Schools can spot an over prepared candidate a mile away and want to hear the opinions of the child not the parent!
Other than that get them to read newspapers/watch the news. They may be asked to identify photos concerning current events or historical figures and give opinions.
There are usually a few Mental maths questions, perhaps a short passage to read and comment on. Having said that, all the things I assumed would be asked (favourite book, subject etc) weren't. If they are asked these questions, try to get them to answer honestly, don't assume that saying they love War and Peace will win brownie points!!
Generally they want to know if your child is a willing and enthusiastic learner and a good fit with the school. Also if this is the school they really want to go to - not just a fall back.
All schools have different criteria and interviews vary. Some interview with the parents, some without. Some are just a general chat, some are more in depth. One thing they all have in common is a willingness to put the child at ease - they are experts at this and if they aren't, is it the right school?