WPOso on "How should I include this PHP file?"

Actually, this tued into two problems, one of which I just solved. As you suggested, I created a new folder - mu-plugins - put my file inside it, and it does autoload.

However, I still need to know how to write includes.

The file inside my mu-plugins folder is fine for creating echo values, but if I want to actually display something at the beginning of an article or at the bottom of the page, then I need to be able to insert an include in the proper location.

Also, I'm eventually going to put some database-driven reference tables on some of my pages, so I'll have to include separate files that feature queries and a lot of styling, etc.

Anyway, what's the proper way to write an include?