unfortch, I don't think I have any tips you won't have heard before, but! For tackling stiffness, I think you really want to focus on looking at the line of action with a pose, and doing the classic quick, timed life drawing stuff is great for this-- like, go as fast as you can, depending on where you're at skill wise. you can look up like, "timed pose drawing" and find lots of sites, and a lot will let you set the timer at different rates. You want to be able to get a pose down gesturally, and then you can build from there.

I honestly think my art looks stiff a lot of the time and I tend to just futz and push around elements until it looks okay! fucking with the skew/warp tools if you're working digitally can be good for seeing how you can adjust the shape or perspective of the piece and then you can draw over your own warped version until it looks right. That's the kind of shit I'm doing, I just don't know if it's actually helpful lol. But I could try to do a small tutorial on how I go about things sometime... maybe...