I often scold the inanimate object that was around a child if they happen to bump or bruise themselves. "Naughty chair! You shouldn't let Billy fall off you! That's not very nice! Let me show you how to be a good chair. You have to be kind to Billy! Billy, let's show the chair how to be a kind one," etc. with accompanying over-the-top silly hand motions and voice inflections and so on. This helps if the child was partly responsible, e.g. if they weren't sitting properly. I'll check in shortly afterwards with, "Billy, are you still helping your chair be a good chair? It's not as clever as you, remember, so it doesn't always know how. I hope you're still helping it!" They sit properly more consistently to this than just asking them to sit up straight or whatever. And it's hard for a kid not to crack a smile when they're praised for 'helping' a chair.
Distraction is especially good for the really little ones, but it's fun for older kids too (even me!)
I've had a few kids who were scared of the dark or spiders or whatever, too. With the dark, if the class was really good, they would all be allowed to sit still while I turned the light off, and then we'd all make owl noises. Because it's framed as a reward, they really enjoy it. With bugs, I always talk about how scared and lost the poor bug must be and how we need to help them get home, and use some tissue to take it outside. This is an adventure and the kids come with me to help find a good spot for it to live. And they all get praised for helping afterwards, of course. Some kids will squeal or cry when they notice a spider on the wall near them, but after the whole class has helped rescue it, then they're generally pretty good if it ever happens again.
Positive positive positive. Don't dismiss fears or pain, but help them through it gently.