I is still getting the hang of the whole Magento layout functionality but have just found a couple of tips that have helped me greatly.
As I understand it the layout handle of and xml file is the node that is called when a particular page is loaded by Magento.
Eg. in a layout xml file your might find a section marked as <mysite_forms_contact_success>
This would correspond to the webpage /forms/contact/success.
You would then configure the layout element for that page within that section of that xml file.
However if you are working on a custom module ore something similar and are unsure which layout your controller is trying to load you can use the following code to dubug:
If you pop that in your controller it will output some info which will tell your which layout is or should be getting loaded.