Over the past 5-6 years I've just been reading up on general construction types of books from Home Depot/Amazon on plumbing, lighting, drywall, etc, as I like to do a lot of the work myself. This lets me design things that are realistic to implement.
Outside of this, most of what I do is search forums like this, browse Houzz and Instagram, watch TV, etc to get a better sense of design trends, ideas, etc. At this point I'd consider my design sense to be more on the practical side, and am working towards doing designs that go beyond practical. Looking at it another way, I think my designs fit 200-300k homes well in my area, but wouldn't be suited to 300+ where things tend to get more stylized. But I'm getting there.
I have learned to use the Chief Architect interior design software to a fairly strong degree, and I have spent a few years now just building random rooms and bouncing them off friends/family. My wife, for example, loves to cook, so I can usually get a good sense of what kitchen layouts work well and what don't.
As far as formal interior design training, I don't have any. I haven't studied the "hard" principles of what some would consider "good design." I'm sure it would be helpful, but I haven't hit a point where I feel not having that has inhibited me. I feel like good design is fluid, in that it's whatever fits the needs of the owner, but is also done with future owners in mind. When I encounter specific questions or want to know what the "right" way is the do something (such as the 'kitchen work triangle' for example), the internet is always there waiting :).
Sorry, this may not help much :).