[Windows RT Development and Hacking] irony_delerium:
This works great for getting both 8.0 and 8.1 on a device - I managed with a 12.3G partition for 8.1, which for the purpose of playing around with it, is more than enough. (I've got a 32G VivoTab RT, so disk space is already at a bit of a premium...)

Of course, you might need to remove some stuff in order to free up the space for the upgrade - after installing 8.0 into the newly created partition and performing the updates (Windows started downloading them automatically, so the disk space was going to be used for a while anyway...) I ended up with about 2G free - not enough for the 8.1 upgrade. It helps to recover as much space as possible - otherwise, as I ended up with in one of the early tries to run the upgrade, it aborted with a message in the setup log about being out of space.

Workarounds:

* Make a restore USB stick and get rid of the recovery partition if you're on a 32G device. On a 64G device, I suspect it's less of an issue but it's still another 4G to play with for a second partition. Make sure you've got your install.wim file from that partition before getting rid of it - that's where your initial OS comes from and you'll need it for actually making the second copy of the OS.

* Disable the paging file. You can (and should) turn it back on later, but for downloading & installing 8.1, it's not needed and occupies a fair chunk of space - the default configuration has Windows tending to use a paging file equal to the size of memory, so about 2G can be released by turning it off. The instructions are floating around elsewhere in this thread for doing this.

* Uninstall all of the pre-installed applications. You can get these back later through the Windows Store if you want them, and they're all removed or replaced in the upgrade process anyway; this got me about 450M back.

From your Start screen: every app that's there, uninstall it. You should be left with IE, Store, and Desktop. Make sure there aren't any hiding under "All Apps" as well.

Once that's done, open powershell as an administrator and run:

Get-AppXProvisionedPackage -Online | Remove-AppXProvisionedPackage -Online

That command will completely remove all of the apps that came with the system. To verify, check (as an administrator) that C:\Program Files\WindowsApps contains only a directory "Deleted".

* Uninstall Office RT.

Again, this comes back in the upgrade, but removing it frees up about 2.5G of space for the upgrade.

As an administrator:

C:\Program Files\Common Files\Microsoft Shared\OFFICE15\Office Setup Controller\Setup.exe

Uninstall all of the components of Office from there.

* Get rid of the Windows Update cache.

As administrator:

rd /s C:\Windows\SoftwareDistribution

It takes up a fair bit of space itself, the update files aren't necessary any longer (the cache will be nuked by the upgrade anyway).

* Finally, run the Disk Cleanup utility and clear out everything. Theoretically, there shouldn't be much to cleanup here because most of the usage that it pulls up is the Windows Update cache, but it never hurts to get back every bit of space you can.

At the end of that, there should be about 6G of free space available (on a 12G volume): the store won't install 8.1 unless there is at least 3.3G of free space after the download completes, and I found that for it to work, you need closer to 4G free.

Once you've finished the upgrade:

* Run Disk Cleanup again and get rid of the old Windows installation.

* Turn back on your paging file.

* If you want any of the apps that were provided with your system, check the Windows Store under "My Apps". They'll all be in the list.

With everything said and done, on 8.1 for a few days, my 8.1 volume has 3G of space left on it.