Always follow up with your catering service well in advance of your event -- a month ahead of time is not to soon, if you've booked them six months before. Even professionals sometimes make mistakes with their schedules, and you don't want to be left high and dry.
When negotiating with a prospective catering service, make it precisely clear what you need of them in terms of food, service, and extras. At the same time, make sure you know exactly what they need from you, in terms of facilities, electrical requirements, and payment.