Some people like what they call stage4/stage5 backup.
In gentoo, stages are pre-packaged systems which are numbered depending on how close to a final working OS they are. While the number grows the "generic" nature of the stages diminishes. In ancient times, people used to use stage1 to install gentoo. A stage1 was nothing but a very basic system with a very basic toolchain. So you have to first, bootstrap it to get a working full toolchain, then start compiling the basic system tools. A stage2 was a bootstrapped stage1. A stage3 has the basic system and a fully working toolchain (and it's the only supported method of installation today). Nowadays, stage1 and 2 are not used by the big public, and are meant only for gentoo developers.
In addition to these, some gentoo users created the concept of what they call stage4 and stage5, which are basically fully working systems tailored to their needs, meant to restore the system easily and on a quick way if something bad happens. There's some info about stage4 and stage5 in the gentoo forum, like for example this one:
However, being as simple natured as I am, I usually just use tar like I'd do on any other linux OS. Simple, easy and portable solution.
PS. I never tried stage4/5 at all.