I'm pushing 40, and getting into the range where ergo issues come up more than I'd like--have occasional RSI issues in my arms and wrists, and my back and neck aren't quite as forgiving as they used to be.
One thing that's worked well for me was moving to Herman Miller Embody chairs at both work and home. I'm not necessarily recommending that exact model. I like them, but the styling is unusual and they're pricy if you don't find a good discount somewhere (hint, ask if you can arrange to piggyback on your work's furniture orders so you get volume discount. I got mine for ~$700 with warranty intact).
What makes them good, though, is that they're built to move around with you so you can fidget properly. The back is built like a spine that flexes, they're shaped to let your shoulders swing back while sitting, there's just enough side-to-side give under the seat to let you shift your weight meaningfully without feeling like it's unstable, etc. Working with the tension settings also lets you find a sweet spot where you're suspended at a proper ~110deg recline by spring tension so you can use your legs to move you forward and back with no effort, which encourages you to flex them. It's no replacement for getting up and walking around, but it keeps you from getting locked into one position and that helps hugely.
I'm sure there are other and probably less expensive chairs out there that give you the same freedom of movement while providing some level of back support. Finding one may make a big difference.
But...beware of armrests. Someone above me mentioned resting the elbow of their mousing hand. Yeah, seems like a good idea, feels better in the short term, did it from the beginning of my hardcore computing career. 20 years later, I've got all kinds of nerve compression issues that probably come from repeated pressure on the radial nerves that run along the outside of your elbows. Plus, if you don't get the height just right you'll shrug your shoulders while you work, which causes another host of issues with neck and back.
Armrests are good--when you can rest your whole forearm on them flat, at rest, with no tension in your shoulders. When you're actually working, may be best to drop them low enough to not be convenient elbow holders though.
In fact, if you can swing it the absolute best position is no armrests at all, or dropped out of the way to seat level (the Embody chair does that too, which I like) and then working w/ keyboard and mouse in your lap instead of on the desk. That'll completely relax your shoulders and keep you from compressing your forearm nerves on anything. When my arms are hurting, I put my keyboard and mouse/trackpad (I use both for different things) on a small portable lapdesk; Logitech makes one that's really light and has a pull-out mouse tray.
Whatever you do, pay attention to the ergos though. You're basically drawing from the "RSI bank" every time you work, whether or not you have any symptoms yet--good habits will get you there slowly, and bad habits will get you there quickly. Make it as slow as possible.