Something I have done is take several pictures of the tractor from all angles to show it's condition and any identifying marks or details, and a picture of the serial number tag to go with it all. With digital cameras, this is pretty easy to do, as if you don't get a good picture, delete it and try again. 

The serial tag picture not only gives me a record of which tractor is mine, but if the unfortunate happens and someone should steal just my tag at a show (happens a lot to the JD guys) I still have some kind of proof what the tractor is. I have a couple that I have thought about making new tags for to use at shows, as the original tag is what sets them apart from the others out there.