One reason people don’t stick with keeping a diary/ journal is the habit of writing everyday, filling at least a page each day, and other rules. A diary is personal. So make it personal and set your own rules and guidelines, subject to change without notice. Make it your own and write it (or draw, scrapbook, etc.) your own way.
Best tip from a SnapGuide post: How to Write and Keep a Diary by Heather Olson-Trow.
I think it also helps if your goal isn’t to get to the LAST page of your diary, but instead, to try to use each page the way you want, which will slowly but surely fill the whole book!
I do write in a journal way, making each entry for each day. At least I don’t restrict every day to one page. I write until I’m done, regardless of how many pages it fills. But, I do have a hard time stopping before the page is full.
So, instead of filling it with more writing, leave space you can fill in later with details like mementos, after thoughts and updates.