I think the structure rates second to the content....
If you know your stuff.. and know every aspect of the topic.. just write down the main points of what you know on the piece of paper that you hand to the examiners.... because no doubt, they will pick and choose what they want to hear about.... and if they arent really interested in the third point you've written then they arent going to ask you about it.... however, if you know all your info really well, you can usually swing their questiong back around to tell them heaps about what your really undersand...
i think what i'm getting at is don't stress too much about your structure.... concentrate more on knowing all of your info and having PLENTY of examples and the structure will sort itself out..
what topics are you doing vivas on??
goodluck
PSJ