I don't think the hack is the index as much as it is the visual skip-list of sorts, and then being able to jump to the page with the content without having to do a binary search of sorts for the page number you are looking for.

That said, physical journaling is a hard habit to keep :) I feel like I've tried every trick under the sun. Bullet Journal seemed to work for a while, http://www.bulletjournal.com/ [no affiliation], but it's still tough to rigorously keep an index up to date when you just want to jot down something quickly.