I'm going with Bunac again next year, just finalising everything up at the moment. I have to say Bunac all the way over Camp America - you pay less and earn more with Bunac, they are much more helpful and generally extremely supportive towards all the people they place at camps. They meet you at the airport, meet you when you reach the USA and have an office in the USA that you can call whilst at camp should you have any problems. I've had friends that have gone with Camp America and had a great time but personally I wouldn't use them.
In terms of experience Bunac seem to be looking for people with relevant experience in teaching or caring for children in a 'professional' environment. For example I coach people 5-22 in cheerleading about 3 times a week. Although I won't be teaching cheerleading with Bunac obviously the skills I have learnt can be applied to different activities. Unfortunately I don't think babysitting for friends/family counts for much, but anyone who is worried about experience you still have plenty of time before next Summer to volunteer with a local sports club or Brownies/Scouts.
Bunac endorse all references received, they won't send your application around to suitable camps if you've got a bad reference because it reflects badly on them. Usually they don't look at references until after they have declared your interview successful though - the interviews I had were basically informal chats, talked over my experience, why I wanted to apply/my motives, what activities I wanted to partake in and why. It was all pretty simple stuff - the questions they ask you are based on the online form you fill in anyway. They also asked to see my passport so maybe take along some form of ID! I was jut super enthusiastic and didn't sell myself short, tell them about all your fantastic qualities and why camps need to employ someone like you.