Every time you run into something you ain't got the hang of yet, study it by doing sketches of them from life as much as possible. If you find you're not getting the folds of the clothes quite right, get your shower towel, drape it over different things and do sketches of the folds. If you have human anatomy not looking quite right, try your 120% to do sketches of skimpily dressed people, from life drawing sessions with nude models, your self in a big mirror, your family and room mate, and lastly, online pictures of people posing. Always study from life as much as possible.
For anatomy, you will also benefit from taking an anatomy book and learn from it - Michael Hampton's Figure Drawing, for example.
For value, hit Ctrl + Y in Photoshop, which should give you Proof Colors - a way to pre-view your piece in grey scale without actually changing your image. So when you Color Pick on your image, even while it looks grey, you'll still get color. In that grey scale viewing mode, check if you have the darkest dark and the lightest light established in your image. Then check if they're being used well to produce focal points in your image.
Here is a quick paint over in which I shortened her thigh, and darkened the background forest to make the glowing elf pop out even more. Feet are not even on the front. Take off your shoe and socks, and look down at your toes, they're lined up very slanted, and vary greatly in size. Study from life, even if it means drawing a ton of selfies.