We're already great with breaking giant template files into multiple smaller files in an /includes/ or /partials/ directory. I'd to talk about how we call those files in our bigger templates. The three options are include, require, and get_template_part(). The first 2 are normal PHP functions, the 3rd is a WP function. (I'm only going to really compare include and get_template_part() though: require is basically include + fatal error, so no real reason to use that by default in our own themes.)
PHP include 'file-path/file-name.php PHP docs
The include function is our default right now. It's got a couple pros over the other options: unlike require, it only generates a warning if the file isn't found, not a fatal error. Unlike get_template_part(), it respects the current file's variables.
WP get_template_part() WP Codex
This is the preferred "WP Way" to ge