I use my photos, currently on Flickr and Google Photos.
My approach is this: At the start of every flight, I photograph the boarding pass sitting on top of the plane safety manual, so I have a clear record of where I was, where I went, on what day, and what kind of plane I took.
I also do this for any kind of train ticket, event ticket, entering museums, concerts, and other venues. It's super useful for organizing my photos afterward, and provides a clear record.
This habit also helped me a lot for work purposes. Due to security background checks, I needed proof that I had entered South Korea. And South Korea does not stamp your passport, they give you a separate small paper certificate to confirm your entry. Fortunately I went through my old photos, and as per my habit, I had taken a photo of the South Korean certificate.