Def agree with the watch your mirrors comments - cars going considerably quicker than you appear out of nowhere.
As per most of the above ensure your car is mechanically ready - good service, good oil, good tyres. As if you were going to do a long road trip in the UK.
Enjoy it! - as much as the ring was an experience never to be missed, I loved the surrounding areas, and the drives through the mountains and tunnel runs e.t.c.
I think never doing more than you feel capable of doing is another good point. If you are only going to do 1-2-3 laps, then do that, and see how you feel. Don't go buying 5-10 laps and assuming your going to do them all straight away. Weather, accidents, car mechanics, how you feel can all affect what you do and don't do. Always be within your own means.
Ryan