Hi Paula,
We booked ours about 14 months before we went.
We did a bit of a tour de USA and some of the hotels we wanted hadn't published their price lists, but the travel agent was really helpful and simply diaried everything forward and contacted us once the full booking had been made.
I totally agree with Angela though - phone or e-mail ahead. Make sure you're dealing with an experienced travel agent - ours was great and knew all the pitfalls of travelling round the US. She made sure we had the best hotels for the money we had to spend and the most flexible flights so we could travel pretty much when we wanted to.
Good luck

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Saturday 16th June 2001 - St Catherine's Church,
Some people are like slinkies, not really good for anything, but they bring a smile to your face when pushed down the stairs