I save an old pair of suit pants that are from a higher weight. I remember trying to wear them at my high weight and not being able to because they were literally like sausage casings on my butt and thighs and I had to suck in as far as I could to button them. I was crushed because they were my favorite dress pants.
Now I can't wear them... because they're too big and literally fall off me. If I'm ever feeling 'meh' about my progress, I'll go try them on and be amazed... because I remember high weight me thinking how tiny they must be.
I also buy ahead 1 pair of my favorite brand of jeans a size down on eBay. It gives me a tangible way to see progress, and once they fit it's my gift to myself to have a brand new pair of comfortable jeans!
It also helps me realize when I've dropped a size, because I'm notorious for not realizing just how baggy my pants are getting. But when I try on a lower size and it actually fits, that quickly shifts perspective.