Listening to music helps sometimes. Often times when I need help motivating myself to do some writing I watch something on Netflix and something in either the music or the plot kind of turns a key in my head and unlocks my imagination xP

For me, music usually helps me many times to get into the groove. Finding the right type of music that i think fits best the idea that I want to work on, it's what usually gets me motivated to work on said idea.
Say that I wanna draw a scene in space, then I search for cool space music. I wanna create an action sequence? looking for cool, energetic music that'd fit an action movie. Or just create a scene of characters hanging out? looking for music that fits, maybe cafe Jazz, or Chill out or Lounge.

Also, sometimes taking a small break from the hobby helps me plan better the idea that I want to work on, instead of forcing myself into it. Like they said, maybe do something else meanwhile, and when you get a clearer idea of what is it you'd like to do on your hobby, when you return to it you'll feel more fresh.

I'd suggest that man, but in general music is what works best for me :)

Old journal, buuuuut: Researching it.
It depends what 'stage' of hobby motivation you mean.

If it's a new hobby you want to get into, then researching it gets me get pumped up to try it. So watching videos of others enjoying it etc. And everyone cant do this, but if you buy something while researching, it helps. So if you want to start art, then buy a sketchbook or a pencil you've never seen (like a 2H or 6H. Many non-artists have never even seen a non HB pencil). This will put you in a mental situation where you have to commit to trying, or you just wasted your money.

If it's a current hobby that you want to get back into, look through your old work. Make a mental effort to remember how much fun it was, and think about what you didnt get to accomplish. Then think about how you are more experienced, and think of all the cool stuff you can do now. The mental exercise can help to excite yourself into thinking about it, and making efforts to restart it.

As a self proclaimed 'jack of all trades' (and ADHD-riddled master of none), i've started many hobbies including traditional and digitial art, gundam models, chemistry, cooking, comics, book reading, RC models, story writing, business startups, fishing, tarot, 'internet of things', tea drinking, acid etching, travelling, and im sure some other things that i've missed.

In each case, I learned about the hobby, did research on how cool it is, then researched its feasibility, usually bought something for 15 bucks (glass etching kit, tea steeper, basic book, etc), then got excited again when it came in the mail and started trying out the hobby. I didnt stick with all of them, but some of them i revisit frequently, and I've learned alot of skills that have helped. Like using glass etching to make an easy personalized candle gift for something like a wedding or home warming. Or making a good cup of tea when i want something complex and flavourful that isnt soda or whatever.

This became a ramble, but take what you will from it lol