I load fare onto what's called a Hop Card, which I then scan at the front of a bus or at a stand at a MAX light rail stop or at a console onboard a Streetcar. The fare is X amount of dollars; at the start of each month I put that X amount, or maybe a little more, onto the card; turns out I don't like when I get the "Low Fare" warning from that console. It's not much of a warning: you get it when you have $5 of fare or less, a day's fare.
I said "sort of" at the start because, as far as I can tell, I can't simply pay $X onto the Hop Cad and designate it as payment for a monthly pass. I use it day to day instead, waiting for, eventually, the fare I've used in a month to reach $X, at which point the console lights up with "Monthly Pass Earned."
It's somehow satisfying to reach "Monthly Pass Earned."
A couple of months ago on a day when I worked so late at work that I knew I'd take a cab home, I took a break from the job for a few minutes to walk to the airport MAX station and scan my card, as if I were riding that afternoon. I wasn't (and didn't), but it meant I'd earned the day pass, and was a little closer to earning the monthly one.
I've found I do similar things on my days off. Today after an afternoon coffee shop-then-library trip, I walked to a frequently-served bus line and rode a bus about 15 blocks, then walked home. (I first considered taking the 17 a ways, but the 17 stop next to the Holgate library branch wasn't going to get its next bus until half an hour later.) Yesterday I rode a bus back to my neighborhood in the morning, after a long walk northwest up Foster. Those two trips got me 5% of the way towards "Monthly Pass Earned"; I'd've gotten 10% of the way if I'd reboarded the buses three or so hours later and earned each day's Day Pass. If I ride TriMet every day in a row, I earn that monthly pass in 20 days; otherwise, with weekends where I might not ride, I earn it in 21, 22, 23, 24, or maybe 25 days.
This might be a hang-up. But I don't mind.