Art is a reflection of Life. Therefore I think Life is the inspiration for art. And if that's the case, I think inspiration is more of a muscle.
The way I ensure inspiration finds me is to have projects I've committed to in a visceral way.
For instance I work art full time, If I don't work on commissions, adopt characters, or other revenue streams, I won't make money and then can't pay bills. Thus parts of making art become "work".
I place myself in such a situation that its do or die. For me, that's motivating because I believe it's all or nothing if I want my career to succeed. I intend to make this work because it is my life's work, nothing else will do. Kind of a 'burn the ships behind me' approach.
This ensures my skills are always at a level where I know I can act on the inspiration and make it become reality. After all what use is inspiration of you are unable to act on it?
Inspiration and storytelling skills in your given medium go hand in hand.
Now when not doing the work, I also make sure to take time for myself to live life. Go see people I love, enjoy exercise, absorb good story content, cook nice food, study story and art, and daydream a lot; all to make sure my inspiration well is full.
I don't think you can force inspiration because inspiration is a thing that happens when disparate pieces of information in your head find new and surprising links. I think it's something that can be practiced by being mindful of life.
Storytellers are the keenest observers of life because that's the source of inspiration. Life is so vast and varied that the content of inspiration will differ person to person, but Observation is key, I think.
As well as knowing your Self. You have to reflect on why you're creating art and story. What about it is important to you?
Self Reflection becomes the "exercise" for my inspiration muscle. Because when I can have faith in my abilities and in my reason for making stories, inspiration has an easier time bubbling up cause my brain is primed to receive it.