In my experience the number one difference between good braids and really good braids is mane prep. A good braider can cover a multitude of mane care sins, but for really good braids proper mane prep can’t be half assed.
You’re looking at length, thickness, edge taper. When the mane is ready, it’s easy to do nice even braids quickly. Speed when braiding down helps keep those braids straight and even, while the mane prep almost guarantees an even length.
Where you put the yarn in doesn’t affect the final braid quality. So if you are counting crossovers, stop. It is difficult to get a perfectly even split on each chunk of mane so I put my yarn in so that the skinny and short sections have the yarn. This bulks up the skinny section and allows me to braid further down than the short section would permit on its own.
When you pull the braids up, resist the temptation to make the top bumps all the same size. The important line is along the bottom of the loops. If any of the loops is shorter or longer than the ones on both sides your eye tends to catch on it.
Be aware of the shape of the mane bed. With few exceptions the mane bed is narrower behind the ears, widens out with the widest point being about one third of the way down the mane, and gets down to the narrowest section in front of the withers. While the ideal is to have the mane an even thickness from top to bottom, that is often not practical as that section by the withers is usually too thin for good braids even when unpulled. Making those wither braids shorter (make the bottom line curve up gently and evenly) helps them to be more stable. Long skinny braids at the withers are easily pushed askew by the reins or rider’s hands. Short braids are more stable in general than braids with longer loops.
A very thick mane needs frequent, regular pulling as doing it all at once means it grows back all at once. That means the braid quality will suffer closer to the end of the season (championship shows).
A long, wispy tapered edge either gives you too much hair left over, or stupidly skinny inches at the end of the braids (long, unstable loops when pulled up). Learn to shorten a mane with scissors so you can create a tapered edge on sections that don’t need to be thinner, just shorter. The technique I use is to grab a few hairs and back comb as if I’m going to pull them, then cut on a 45 degree angle at my desired length. When presented with a mane with a ragged edge (usually from careless braid removal) I will clean up the edge with this technique before starting to braid.
Evenly spaced braids are ideal, but when the mane isn’t an even thickness I find it looks far better to have even braid thickness with closer spacing in the thick spot than it does to have a bunch of chunkier braids.
Practice is a huge help in improving your skills. Having a good mane to braid makes everything even better.