A lot is written here about managing statamic in what seems to be a personal environment, having the luxury of no-one messing with your content, using high end dev tools like git to manage and automate deployments of locally changed and managed content. However in the real world most clients can not begin to adopt such a workflow, and if you have a management or maintenance contract with your clients some sort system needs to be worked out so here is ours.
Firstly let me say we are lucky to have most clients running on our own, not shared server environment, so we at least know what versions of php/apache etc are running and can upgrade where needed.
Firstly once each new site is deployed, we strip out locally all but the specific files for that client ie the _config, /users/, /_fieldsets, /add_ons etc and obviously the theme files. These are then put under version control in their own repo.
We also keep a repository of the "core" statamic files, basically the _apps and _admin folders plus a couple of bits and pieces.
When a new version is released into the wild we only have to deploy the _apps and _admin folders to the central repo which then pushes out to all our statamic sites via dploy.io. Voila a one click statamic update across multiple sites! The only provision is we do trawl the changelog and run all folders through a diff tool to make sure no important changes to for example config.yaml has been missed, if so we have to do this bit manually, but its quite rare.
Lastly all client sites _content folders and backed up and put under git once a week, as well as our server backing up to Amazon nightly.
I hope this helps someone or inspires a better way, I doubt our way is perfect or even best but it works. For us as a small site specialist statamic has proved to be the perfect tool for everything we need, and has managed everything we throw at it.
Going for our own server some years back and building hosting/maintenance/management contracts into each site has now grown into a very good regular monthly income stream, allowing us to much more easily manage cashflow than just relying on design contracts