Zend certified PHP/Magento developer

Best way to override custom theme

I bought a custom theme for my Magento store and want to customize it slightly.

Customizations mean that I will probably update layouts, templates, and other files that are available under app/design/custom-theme.

What is the best way to customize these without losing changes when a future theme update is performed? I am thinking about creating a child theme, but I am unsure if this is the best strategy.