This. If Luna was going to wear a dress, or be humanized in general, THIS is what she'd look like. It's so deceptively simple! I love the sheath dress, especially with the starry night cape. And those different colors...gorgeous. The jewelry she's wearing is heavier than what most people design her with, but I love that! The pendant and armbands suit her. They're more.....warrior looking. Amazon like. It suits her. Feminine, but tough and dear jesus, her hair is stunning. It looks beautiful against her wings.
The best advice is: practice, practice every time you can and observe in detail other artist work. In addition to a skilled hand, the details are what65 make stunning works. And of course, try every tutorial you can find here in deviantart, they're very helpful!
Wow, this looks spectacular! I am really incredibly impressed by this, the colours are so vivid and fitting and I love the stars in her hair! The wings, especially, look incredible and I also really like her tall figure and expression.