In step number 6
HERE, during install if you choose "Something else" at the bottom, you can setup partitions yourself. I did 15 GB for ROOT (which is way more than enough), I usually do equal to or 1.5 times the amount of RAM as a SWAP partition. And then the rest I set as /HOME. Then next time you reinstall Linux you choose "Something else" again and choose that same partition as ROOT and tell it to format it. Same one for SWAP. And same one for /HOME but DO NOT format it. In the reinstall if you use the same user name and password the permissions will all still be fine for your documents. Most of the time I'm using Ubuntu based distros. I haven't tried it going to a distro based on a different Linux but I would think it would still work. Usually the only thing different directory wise on distros is system side, not /HOME side.
Edit: Just remembered. I have done that when going from Crunchbang to Linux Mint and it worked fine. But then Crunchbang is Debian based and Ubuntu derives from Debian too.