That said, my experiences over the last few years have qualified me to add a few more dos and don'ts to Damian's list...
Firstly, do remember to pay your visiting author. I haven't had too much trouble over this, but there was the time I called a school to check that they'd got my invoice, as it had been a few months now and I still hadn't been paid... to be told, "Oh, we thought the bookshop who organised the event would pay, so when we got your invoice we just ignored it."
Never ignore the invoice. Never. Query it, by all means, but never just ignore it.
Secondly, almost as important as getting the author's name right is getting his or her profession right. It's immensely irritating to find that the teachers are expecting a 'storyteller'. It's not the same thing. Fortunately, the children won't be expecting a storyteller, because if the school's so disorganised that the literacy co-ordinator books an author and then tells her colleagues she's booked a storyteller, the teachers are unlikely to have thought to tell the children anything at all.
Thirdly, if the children are enthused and inspired by the author visit, some of them will need a focus for that enthusiasm and inspiration, so do promote the end-of-day book-signing. You may be surprised by some of the children who are most keen to get their signed copy.
All that said, though - it does work both ways. I've heard so many authors complain - justifiably - about ill-treatment at the hands of schools, that it rather took me by surprise when a teacher who'd booked me for an event this week took a minute for a little moan about some of the authors she'd worked with. One, apparently, rejected everything she was offered for lunch, so that something special had to be made for her, and needed constant attention throughout the day. Another followed a stream of emails with a forty-five minute phone call about her requirements for the visit.
When it comes down to it, I suppose - at the risk of sounding middle-aged - it's simply a question of manners, on both sides. Whether you're an author or a teacher, think about the needs of the other party in the agreement. And remember that the main beneficiaries of any author visit to a school should be the children - and this is most likely to happen if the author and the school treat each other with respect and courtesy.