If you know where they need to take the floors up you could get the carpets up ready but delving under the floors isn't normally the first job. It's probably better to make sure they will come up easily.
The thing that I find most useful is when the customer has set aside somewhere for us to keep our tools and materials and work from. That's especially useful this time of year.
Make sure you've got some time set aside for when the guys turn up so that you can walk round the job with them and make sure that your expectations are the same as the job they are going to install. It's very easy for both parties to interpret information in different ways so this ensures the job gets off to a good start.
Most important of all, have the kettle on when they arrive. Even if the job is a complete pig it always runs better if you get off on the right footing with the customer.
Mike