The advice to use a larger brush didn't come from me, but came from John Singer Sargent. The quote is something like "always use a brush one size larger than you think you need". I spent a lot of years trying to be Sargent, so I took that advice to heart. I forced myself to use really large brushes for everything except the details. This may be part of the reason I gravitated towards flat/square brushes: a square brush lets you cheat a little. I figured out how to use the corner of the brush for smaller details, how to hold it horizontally or vertically to get different sized strokes, how to use it like a calligraphy pen, etc. 

For a handful of years I worked in a semi-strict routine. Once I had my composition loosely drawn in charcoal or thinned out burnt umber, I'd start with the biggest brush I had and loosely block in everything on the canvas, covering up every bit of white that was showing through. From there I'd use that big brush to finish as much of the piece as I possibly could. Only once I got to the point where the big brush couldn't refine it any more would I move on to the next smallest brush. I'd go from a range of 2.5" wide down to liners for just the last few details. I don't want to imply that I was punishing myself with strictness, I developed that system because it was fun for me. I liked the ritual. I liked the challenge in the same way I like putting a video game on a harder level. I liked laying out the brushes in order, I liked the feeling of putting the bigger one away as I moved to a smaller one. It was a lot of dumb little pleasures I let myself indulge in.

I also don't want to imply that that's even necessarily the best or only way to get to the same results. I don't actually work that way anymore, but I can achieve nearly identical visual results. What I do now is block in with a bigger brush, but then use a smaller brush to work out the edges of the bigger shapes. Once I noodle with it for awhile with a small brush, I go back into the wet paint with the larger brush and simplify everything to loose, flat, larger shapes. After that, I noodle some more with the small brush and do that over and over till I'm done. Incidentally, this is where the technique of scraping emerged: after a few hours of noodling, the paint would get thick and gross and the colors started getting muddy, so I'd just scrape it down to get the paint out of the way. I learned pretty quickly that if I scraped gently, and smoothed everything out with a fan brush, the work I did mostly remained, even without most of the paint. Plus there was the added benefit of the really thin, translucent shadow areas which I've grown to love.

I use square brushes now, mostly, but I spent probably a year and a half using only round brushes. Again this was in my pursuit of Sargent; he used round brushes so I had to use round brushes. After giving it a fair shot with the rounds, I decided I liked the squares better and went back to them. I realized how little difference it ultimately made to the final piece. I couldn't tell you which paintings I did with rounds or flats, just from looking at them. And as I was transitioning from one to the other, there were plenty of pieces where I used both.

But this adventure into round brushes wasn't wasted effort. When I went back to the squares, I found myself using them differently. I wasn't thinking about it really, but the way that felt right had changed and my work was better for it. It also taught me an incredibly valuable lesson: the brush type had little to no effect on my style. Even with different tools, different techniques, there was something that was definitely me that showed up in the work no matter what tools I was using. My style and technique have matured and refined over the years, but there's still something that's consistently “me” about it.

And all of this is to say that I don't think you necessarily need to do any of these things. Rather, I point all of this out to illustrate what I think is the most important advice I can give: find a way that's fun for you. Maximize the amount of pleasure you can get from your process. If you're a person that likes little rituals, go for it. If you want painting to be a place in your life where you can indulge in rampant chaos, dive in, just be sure to use a drop cloth.

I truly truly believe that the difference between good and great painters is passion for the act of painting. As rewarding as it is for me to complete a painting, my goal is not to have a bunch of cool paintings. My goal is to spend time painting. I think this is a large part of the looseness of my work, as well: since I'm just trying to have fun, I don't worry so much about whether or not it's good. If it's bad, I'll just scrape it off and have fun trying again tomorrow. Or maybe I'll just do a different painting.

To make a good painting, the only skill that matters, really, is drawing. If the artist is competent enough to make a convincing drawing, the painting will be just fine. A great, painting, however, is made by someone who loved it while they were doing it. I can see a clear difference in my own work when it's something I'm into or if it's something I'm not. I have a 6 or 7 volume set of the complete works of John Singer Sargent, and there are definitely paintings that are way better than others and I suspect that often, the difference was his own emotional investment.

The best practical advice I can give is to try a bunch of different things, keep what you like, discard the rest. Your personal style will naturally grow from that process.