Well, for me there's two different approaches (that differ widely in goals and other stuff.
I took a creative class when I was a small morbid fruit, and the instructor's philosophy was that you should write anything when you've hit writer's block. It doesn't matter if you think it's dumb/stupid/bad; it's important to get your fingers/hand moving. As a result, in that class we did a lot of exercises like drawing random words out of a hat (i.e. pineapple, platypus, nuclear) and writing for 30 minutes.
We also did 5-minute on the spot writing, where you write anything and don't look back.
Of course, the idea isn't to write something that you would publish, but it's a creative writing exercise. You'll end up discarding 99% of the things you write, but that's also the point. The point is to emphasize that you don't have to be perfect your first time writing something, and in many circumstances half of what we do is getting our first ideas out onto a page.
On the other hand, if you hate the idea of writing things that you don't plan to publish... well... you'll have to be more direct and just jump straight into your main story.
I usually lie in bed for a bit when I try to figure out what the next chapter will be.
But honestly, I think it's best to write when you're excited. If you watched an anime and got a burst of inspiration from that, I think it's good to ride any bouts of inspiration that you get!