I am starting with Accenture in July and recently passed both interviews. In fact, I also got offered jobs with Deloitte Consulting and Oracle too and got passed the first stage with IBM BCS before dropping out. What I would say to you is BE PREPARED. Read this forum and others, phone them up and ask questions if you don't understand. Make sure you read their website and have a good idea of what they do. Know the company structure (Operting and Capability Groups).
The first interview will be a 1-on-1 with an HR person or a Manager. I had a Manager and it lasted about an hour. It was very relaxed and she allowed me to talk quite freely. The interview will be competency based meaning you will be asked 'behavioural questions' such as 'Give me an example of a time when you used your initiative to solve a problem'. Do a search of the web for the types of questions you might get and the areas the questions might cover. I found this really useful. PREPARE SEVERAL ANSWERS! This will make sure you don't have to think on your feet too much during the interview - just recall the answers and examples you thought up and fit them to the questions you are asked. The interviewer will write stuff down while you answer - try to use this to your advantage by pausing the let them write but also giving you time to think ;-) Make sure you answer succinctly but completely as you will be marked in each area and its all about getting enough ticks in enough boxes.
You will also be given a mini case study but don't worry about this as it is really informal and is just 1-on-1 like the rest of the interview. Mine was asking about how a hotel might retain more customers so I talked about loyalty schemes and CRM systems using the Hilton chain as a real world example.
Most importantly just try to be calm and be really enthusiastic about the process - this will come across really positively. Don't be at all negative - even if your journey down was a nightmare. Remember you are being evaluated from the moment you walk through the door. Finally a firm handshake should start and end the interview well.