Hey man, just spotted your excellent question!
I'm also a hobbyist, and now that I found a new job, my drawing time is very limited. I can't say I'm practicing this as effectively as I'd like, but there is something valuable in drawing every day for at least 20-30 minutes, even if it's just doodles. This keeps your muscle-memory in shape, but it also ensures you keep observing your surroundings like an artist. A lot of my improvement is done by looking at people and places in my daily life, and noticing interesting highlights on their cheek, colors, shapes, and whatever is appealing to me that I can transpose into my art.
As for the most effective practice, it's also the most draining way, so make sure you've got energy and fuel in your tank if you want to take the highway to mastery. The quickest and most effective way to improve is to copy and imitate your favorite masters. Take a piece you love, and copy the shit out of it. And again, if you're not satisfied by the first copy.
While doing this, pay attention to the light and shadow shapes, overall values, composition, design features or elements, perspective... Anything you notice can then be analyzed and integrated into your own toolbox.
A 1-2 hour study of your favorite pieces per day will absolutely improve your rendering, stroke economy, and composition skills to the point where you'll be doing your own work much faster and more confidently. Everything you do will now be a patch-work and remix of things you've painted (and copied) before - by now you can experiment and change things up.
Hope this helps! Cheers!
