Or, as another method for data caching and thereby saving the custom field values, you can go with Steve's own method, described here: http://scn.sap.com/community/gui/blog/2014/03/03/data-caching-in-personas-scripts
I find that doing this may sometimes be simpler, because retrieving and re-populating the custom fields can be easily automated, tied to the OnCreateHandler of the page. This is especially so if you have many different navigation paths to get back to your screen with the custom fields.