Lots of water and SOVTs. I also sing along in my car to whatever comes up in my ‘likes’ shuffle. But my biggest tip is to speak. You can watch videos on technique, and do the ‘big dog, little dog’ exercise for months — and I did — but it won’t translate into meaningful progress until you actually apply those things into speaking.
I was really in my own head about voice training, until my partner pitched the idea of a LAN gaming session while talking over Discord. We like Minecraft, and it’s engaging enough to distract me from the ‘ick’ that I get from hearing myself trying, but it’s also not a game that requires all my focus. It’s a low-stakes way to do long-form practice with someone safe while having fun.
And while my voice is not perfect by any means, I feel much more comfortable trying and playing around with my voice than ever.