OK, my method may seem unconventional to some, but I've tried a lot of methods, and this one has really worked out well for me.
I create a big partition for documents, pictures, music, etc. For my own historical reasons, I labelled this partition "shared" (because it gets shared with any other OS I might install. I mount this at /mnt/shared, and change the owner to myself (chown me:me /mnt/shared).
In this partition, I create directories such as "Documents", "Pictures", "Downloads", "Music", etc. Once this is mounted correctly, I then make symbolic links of the well-known directories seen in "Places" in my home directory, and I delete each of the the still-empty directories in $HOME and immediately replace them with the symbolic links, named exactly the same as the original directories. Doing this immediately allows them to obtain the special icons automatically. If not done right away, they need to be set by editing the file ~/.config/user-dirs.dirs, which isn't very hard, but it's an extra step.
Once this is done, they appear to be exactly like the original directories, and work the same. But, they could be used for a different OS in the same way, even Windows, if needed.
I think this is much easier than setting up a separate $HOME mount, as you can't easily reuse the home directory for a new or different distro. I've had many problems in the past trying to reuse my home directory; this is much easier, as it isolates almost all the files you want to save.
Edit for more elaboration as requested in comments
I often like to try new distros, sometimes even off a live media. In order to evaluate a system, it's nice to have your documents and other media files in order to actually use the system to do your normal work. But you don't really want it to write a lot of files into your home directory, since you may decide to abandon it. So I let it create a new home directory in a new partition, and mount the big media files that are separate, where you might even want to save files that you use on the new system.
Also, when I install a new or different distro, I like to use the same username and password, so I don't want my working home directory to get screwed up by a possibly temporary system. But I'd do it even if I used a new username just to keep things tidy.
The "Places" I mentioned is shown in Nautilus, on the left where it shows "Places", "Devices", "Bookmarks", and "Network". I don't know if it's called "Places" for other file browsers or flavors. Also, in the home directory, those folders have a special icon.
