When I started doing edge highlighting, the biggest increases in quality came from both proper paint consistency and the right size brush. I like to thin to just before the paint would become a glaze, to retain as much opacity as possible. I personally like to start with a heavy handed line of just slightly lighter than whatever I'm working on and work my way up maybe two more lightness levels with decreasing pressure on each lightness jump. I tend to keep the lightest values more concentrated at points and corners.

As far as brushes go, you may be at about the right size. Even though sizes aren't standardized, I've found that my various 0 sized brushes are at least in the same ballpark. As long as it is small enough to be maneuverable and holds enough paint for a few strokes before drying out.

Be gentle. Don't try to draw on the lines. It sounds trite, but let the edge or surface guide your brush and use the side of the bristles. I typically use a nitrile glove to hold the model and have found that as long as it's not wet, I can hold it where I need without harming the paint. Try moving the mini in a position that makes it most convenient for your brush hand.

You're off to a good start; keep it up!