How do I make my art look less flat?

[AVXP]

I think those that asked you to work on the lighting/shadows were right.
Everything else, is basic drawing concepts, not really related to a "flat" look.

Every image has a light source.
You should imagine a strong one somewhere in the scene and use that as a guide.

[JoshRaed]

So here’s the pose from other angles. What would the other arm in this shot be doing?

[Kelheor]

It will depend on what you're trying to go for. I will repeat once again, you have to understand what you're doing here with this particular pose. Right now it's the guy flying and casually trying to pierce something with his hand.
I double dare you to stand in front of the mirror and try to pose like this. Using real life references will help you much more than static 3D models with broken arms.
@Katzalcoatl showed you a few examples.
And in any case, this has nothing to do with the original thread topic, so I will stop here.

[JoshRaed]

The thing is this pose is something he’s doing in midair, while leaning so far forward he’d fall on his face if gravity worked normally, so it’s not easily repeatable.

[JoshRaed]

Is this better? I think the arm sticks out weirdly in this pose.

[Katzalcoatl]

I see the problem. You are just moving the arm without really moving the torso. I would honestly just use superheroes or the poses I added before as a reference.

But if you "must" use 3D model references then here are some examples of what poses you could use/do.