It would depend on the nature of the work. When working at home for private clients, I start by opening a folder for the client with a bunch of standard documents, which I customize, such as contracts, invoices, reports, requirements, testing, code repository, etc. As the project develops, I add/modify the directory as required.
If I had to go back to a project, I would reopen that directory, and for any non-common components, create a new directory. For example, if my client had a web application built, and now they need a second application, I would use the same directories for invoices and contracts and create new directories for the code base, requirements and testing.
In terms of backup, I archive the work at any point where I've reached a milestone, with the exception of code, which I back up daily at a minimum. At the end of each project, when I close a contract, I take the entire directory and compress it and store it on a remote server.
