Make sure your bags have TSA appeoved locks. Other wise don't lock your bags - the TSA will bust them open and leave a nice letter inside saying that they are not responsible....
When completing your visa waiver from and you get to the part asking for your US address write 'in transit' across the lines. If you leave it blank the Border Control people will ask you loads and loads of irrelevant questions. Have your onward boarding card ready but do not present it unless asked - presenting when not asked for just confuses the agent.
Just remember at all times that the US have no real concept of International Transit and in effect you have to 'land' in the US and leave again. Patience and politeness at all times no matter what!