Every project will need an update or a tweak in the future. One way to be prepared is to keep good records after the fact. When a client calls and asks for an update, you will then have everything you need when you need it.
Everyone will have a preference of how to keep records and there is truly no right or wrong way. I choose to keep a folder on a secure server that houses the original designs, project plans, and functionality files. Paper records are fine for some information; however, electronic versions of the records are searchable in most cases. Make sure to keep a backup of all files in case of a server failure.