Just to add my 2 bob worth -
I did my first ATB last year. Although I have ridden for many years, I had never done an ATB before. My rationale had been that I didn't want to fork out hard earned $ only to have it rain on the day (I detest riding in the rain). I figured that since the weather in 2003 was so bad, 2004 would be good - and it was. I'm not doing it this year in protest of the price rise.
Best piece of advise - keep a close eye on the weather forecast leading up to the ride. The weather will dictate what sort of bag you will need to take. With the weather last year, I didn't need to take extra clothing. But I did have to make sure that I had somewhere to put my arm and leg warmers after I warmed up (if the day is going to be fine and sunny, the morning will be chilly).
For last years ride, I took a bum bag. In addition, I utilised my jerey pockets. I took a puncture repair kit and two spare tubes (both of which are useless unless you take a pump - frame mounted), extra food (from memory - 2 bananas and 2 jam sandwiches cut in half), two bottles of water, money, mobile phone, atb passport, sunscreen. I think that was about it.
Try to get to the ferry as quick as possible. I rode Docklands - Sorrento. Got to the ferry at about 8.30am and only had about a 1/2 hour wait.
Geelong back to Melbourne can be a bit boring and a head wind makes life a bit harder. Get into a pack and start chatting to people. Take turns at the front, giving each rider a rest when they draft (if you are not a pack rider, practise before hand - very easy to cause an accident).
Its a good idea to set yourself a time goal but overall, have fun. I enjoyed the whole day, from the amazement of seeing thousands of red, flashing rear lights at the start, to the ferry crossing to riding over the Westgate bridge. The end is also a good atmosphere.
My time for the ride: 7 hours 17 minutes, not including ferry but including all other stops (toilet/water stops, traffic lights etc).