Currently, you can back up any particular app instance (grain) by clicking "download backup" in the topbar -- this gives you a .zip of its data, which you can then re-upload to any Sandstorm server. For full-system backups, you'll need to use a standard Linux filesystem backup solution, configured outside of Sandstorm. We (Sandstorm team) plan to add the ability to configure full-system backup through the Sandstorm UI in the future. (Of course, if you use our managed hosting, we are making daily backups.)

I'd love to see integration with uploads of backups to Backblaze B2!

<vaporware>Indeed, the plan is to provide an API such that apps can be written to push backups to different providers. The API will only provide access to already-encrypted data, so you don't have to worry about the app stealing your secrets.</vaporware>

As kentonv mentions, right now that's not fully fleshed out. I do download backups of important grains somewhat regularly, but I'm currently using their hosted service until I'm ready to run my own. (Literally just got a server to play with recently.)