I've been dealing a lot with this topic lately, as I'm working on my second feature-length film mix in as many months. When you're working on mixing levels for a project that's 90 to 100 minutes long, over the course of several days or even weeks, it's easy to lose track of the loudness of the mix over time. Having a properly calibrated listening environment is key, but having tools to help you keep track of your overall loudness levels over time can be a great tool.
In conversation with Andy Kirshner, friend and U of M Professor, and his wife, the topic of mixing came up, and how modern films have hundreds, sometimes literally 1000+ tracks available, and how these colossal projects get mixed, and this article from Walter Murch at transom.org came up, which I had never read. I'm so glad I have, now. I'm going to quote a rather large section from the article, which I hope will lead you to click through and read more. It's certainly worth it:
Oh, and by the way, in this one instance, READ THE COMMENTS. Murch himself engages the readers and goes into more detail about many points. Who knew a comments section could be not only readable, but informative and enjoyable!
*article thumbnail photo of Walter Murch mixing Apocalypse Now from rogerebert.com