1) Listening to great pianists or child prodigies playing the piano.
This definitely, especially for a young adult like me started playing piano, I always find the urge to sit in front of the piano and start playing everytime I watch some child playing great pieces on youtube. And then there's the great pianists too like Horowitz and Argerich who inspired me to play a lot.
My friend motivates me too. We started at the same time and we check each other progress from time to time so that no one is slacking lol.
Other than that, I set myself a far away goal too for example I want to be able to play a particular piece (that's ahead of my current skill) in say hopefully 3 months ahead.
I also make it a point to play the piano everyday, even if it's as little as 15 minutes (but usually more than that).
I think the main part is just to find a practice routine that's interesting and not tedious. Once I feel I accomplished something or made progress, I would feel wonderful and that's what keep me going. For example, if I find the pieces I'm practicing starting to get boring, I will start learning another new piece. Then on the next day I could alternate practicing the old pieces and the new one. Also since practicing scales can get a bit boring and tedious, I insert them between playing my pieces.