Stash template inheritance
The Stash template inheritance pattern allows you to build a base "skeleton" template that contains all the common elements of your site and defines variables and blocks that can be populated with content or optionally overridden by child templates ("partials").
It can be combined with the powerful routing add-on [Resource Router] (https://github.com/rsanchez/resource_router) to allow you to create simple and modular templates that are easy to maintain.
An illustrated introduction to the concept can be found here:
A full implementation of this pattern, with working code that you can dissect, was created as part of the EE Zen Garden challenge.