Well , i think it will have to be cookies!!
1- You will need a php function to check if the cookie is set
1.1 - If there is no cookie set continue
1.2 - Else (if there is a cookie set, then use the value in the cookie and fill out the form fields with the cookies contents)
2 - You will need another php function for when the user clicks on the save button
2.1 - You will need to check if the fields in the form are not empty, and add those values to the cookie, if its empty dont.
I think you will be able to find out loads of info about this on the net, but Javascript will be a more appropriate language to use i recken!!
I hope i helped!
thx
peace