The wonderful folks at Radish Lab put together a mini NationBuilder guide, specifically around building familiarity with the theming framework. It is, in their words, "chock-full of useful tips and tricks to get you started". It's true - the guide is an excellent resource for designers and developers that are new to Liquid, need a refresher, or are hunting for savvy techniques.
The coolest part of this guide (in my humble opinion) is the fact that it's open-source and lives on GitHub. It's designed for collaboration and relies on community to keep evolving. Very true to NationBuilder form.
This actually started as an internal reference project for Radish Lab, a way for them to learn and document designing themes on NationBuilder. Although it accomplished that goal entirely, they had a brilliant continuation idea: offering up a well-designed foundation for others to contribute their clever insight. It's the quintessential wisdom of crowds.
Be sure to explore the guide and certainly don't be shy about being part of its growth!