Other things that work but are a little more difficult to do often:
PACT! It gives me a kick in the butt just about every day 
No I'm joking of course. More seriously, I'm probably in an entirely different league than you are @ranjit. I don't have this super serious motivational drive to get really good really fast.
In fact, that has been a main motivation: the fact that I'm not having to qualify for the pianolympics any time soon. Easy does it so I'm setting realistic expectations and goals.
Any time I feel that I'm hitting a solid wall, I turn it down yet another notch and go back to earlier pieces and marvel how easy they feel now.
In summary, rather than forcing myself to push through, I get my motivation from realizing it's just a hobby and no one is standing over me with a whip.
Maybe not exactly the answer you were looking for, but eh, you asked 
Sophia In summary, rather than forcing myself to push through, I get my motivation from realizing it's just a hobby and no one is standing over me with a whip.
I used to push myself to get better just like ranjit (maybe to a less extreme extent) and sure I achieved quite a lot of improvement but it burned me out. Now I'm like you and just play for enjoyment. It certainly does help that I've been doing this for a while so I can already play some of my dream pieces 
but I don't stress about it if some pieces remain out of reach.
Burnout doesn't have to be because of the piano or piano practice, when life pushes too hard, people tend to pull back a little.
Tomorrow will be 2 weeks since I stopped practicing every day. I've sat down and plinked on the keys twice in those 2 weeks, but haven't really done anything. Yesterday I played through Snowfall (my latest lesson piece) without warming up because I just didn't feel as if I could do that and still play. Then I turned the page in my method book to look at all the theory I need to learn before the next lesson piece. I closed the book within 30 seconds because right now I have no spare personal energy to devote to that. Total time on the bench = < 3 minutes.
This isn't the first time going through this for me either. What I've learned is that if I keep the dream alive, eventually I will return and play/learn more.
You will too. But only if you want it.
If I don't want to play, I don't play. Every now and then I just don't feel like it. Usually if I take a day off, by the next day I am chomping at the bit to get back to it because life just doesn't feel right without music every day! But I may play stuff other than my current lesson pieces if that feels too hard. 
Hi
Well perhaps I'm unusual, but I just don't have motivational slumps. And I think it's the sheer variety of what I play that keeps me interested, and has done since I was about 18. Currently I'm having classical lessons (studying ABRSM grade 8), playing in a big band every fortnight, playing in a pop/rock band, hosting a Jazz jam session once a month at my house, and attend a fortnightly pop/rock jam session. It helps being retired!
Also I've always been realistic about my ability. I always knew I'd never be a very good classical Pianist, or Jazz Pianist, or indeed a very good Pianist in any genre. But I've achieved more (I won't bore you) than I believed was possible, when as a 9 year old (in 1970) I first started playing. It took me almost 10 years to find a way into the Piano (in non-classical), and in that period I certainly had slumps, but since about 1980 It's been an important, and continuous source of pleasure in my life.
Cheers
Simon
All round average Jazz, Blues & Rock player.
Currently working towards ABRSM grade 8.
@Simonb I definitely get that and I don't think you are unusual. My father started playing when he was a child (different instruments, none of them piano). Of course he became a professional (solo performer, in an orchestra and later full time teacher) and then when he finally retired, he kept on playing for pleasure. Even when he became too sick and too weak to do anything else, he still managed to pick up his instrument every single day until the day he passed. Some people are simply born for music 
Try working on something new - and not too difficult that can provide more short-term rewards.
If you can figure out what is causing the lack of motivation, that will be a clue about how to proceed. Are you frustrated with lack of progress? Are you working on too many pieces? Are the pieces too difficult? Are you tired because of other demands, e.g. work? It sounds like a good time to step back a bit from piano, take a break, and assess how you want to move forward. There’s nothing wrong with taking a break from piano.
pseudonym58 This 
I'll go purchase something very different to my usual pieces at sheetmusicdirect or JW Pepper. There can be lots of arrangements to choose from, starting at very easy to more advanced, and sometimes an official score. I currently have a Hozier piece to turn to when my baroque pieces wear thin. Certainly cheaper than a fresh tuning, never mind a new piano or partner 
For me, when I’m not excited about playing but feel I “need to,” I ultimately don’t rely on motivation and instead treat it as a clinical, straightforward activity similar to brushing my teeth.
It works for me for things like healthy diet, exercise, etc. as well.
You start by taking a break.
If the motivation doesn't come back by itself, you think about what would be the most pleasant thing to play/practise on the piano, and you do that.
If you are still not motivated, time to analyse what it is that made you lose your motivation. Were the pieces you practised not interesting enough, were they too easy, too difficult? Did you press yourself too much? Or isn't it anything piano-related - is your life away from the piano too demanding?
*
... feeling like the pianist on the Titanic ...
Finding new & interesting pieces to play including going to a piano store, downloading sheet music online.
Usually, it's a deadline. I HAVE to learn a particular song or songs for a particular gig.
I'm in a place I haven't been before. For decades, I did a lot of church work and could run nearly on "autopilot". I knew pretty much all of the songs that would be thrown at me, probably played them hundreds of times. Would use most of my creative stuff for offertories and postludes. But now, the church work has ended and I've entered a realm where what I play is generally paid work. No, I'm not getting rich. But it's not just for fun anymore. It's a different dynamic than I've lived with previous to the past two years or so. (Church work was strictly volunteer other than the occasional "special occasion" when they'd give me a little pittance. It's not that many years ago that I realized that some church musicians actually get pay for their work. Anyway, left that world behind and have no intention of going back.)
There is a local church planning on a fundraising event. A friend asked me to play some music as a duet. I promised to do up to 5 pieces. Learning them in a month is the motivation.
A lot of people would learn assigned pieces for a teacher. We talk about bringing pieces up to the performance level. The majority are not planning to perform even for relatives & friends. On the other hand, you can always submit a video / audio recording to the online forum. Even a short piece would give you the incentive to work harder to get the best possible sound.
thepianoplayer416 A friend asked me to play some music as a duet. I promised to do up to 5 pieces.
The few times I've been asked to do something like that and presented with the "sheet music", I got to the point where I declined. If I wasn't able to "feel the music" such that it just came forth naturally, it was nigh unto torture for me to learn stuff I usually didn't even like note for note and have it sound good. It's not that I couldn't, but I REALLY did NOT want to.
I'm still shy away from printed music. I'm not exactly sure why. I can read it. Then again, there are some music styles that I can play but loathe to the point I can't even listen.
Guess that's what happens when ya get old and grouchy like me. LOL!!
Music in church is mainly to support a choir or 1 or more singers. Unless it’s a talent show, the music would be religious with the focus on the singer(s). As a performer you may be allowed to choose your pieces depending on the occasion.
I belong to a music group. I don’t always get my choice of pieces but keep up with practice. Everybody in the group enjoy music & playing together despite not liking some of the pieces.
You come across unfamiliar pieces all the time. Even when learning with a teacher, you don’t choose many of your pieces. If you only play the pieces you like you wouldn’t be doing many of the teacher’s pieces.
Somebody online brought up an interesting point many students quit piano because they’re learning pieces they don’t like or not related to their life experiences. If corse there many different reasons why someone would quit including being forced to learn as a child. Part of the learning process is to try different style & genres of music. You’re not going to like all the pieces.
I don't gig and have no musical commitments. I just play for enjoyment, or, creatively speaking, because I can't help myself. And there are times when I don't feel like playing piano, either because I'm bored with what I sound like, or just feel like "why bother?". I also play guitar, so sometimes I get involved with that at the expense of the piano. Usually, I gravitate back to the piano, maybe because I've had an idea, or heard something that reminds me of something, or something else. But what really makes me want to sit down at the keys is listening to Bill Evans. Even though I don't play like Bill, and never will, sometimes I just feel like I want to make some of those sounds, the feeling that comes from creating a voice, maybe even a single chord. The goal at that point isn't to try and play like Bill, but to try and capture the essence of how Bill expresses a melody or phrase or combinations of notes over a chord. Once that impetus strikes, I'm good for while, until I get bored with my playing again.
