How do I archive inactive projects including history?

• Select one computer as your archive.
• Join all projects from that computer (it's enough to join as observer--no license needed)
• Ask the member with the longest history in a given project to export his or her history
• Import this history on your archive computer
• Have everybody else defect from the unused projects (before the last person defects, make sure the archived version is selected as administrator).

Any time you want to revive an abandoned project, simply let people join it again. This is also a great way to back up your projects.