It's hard to give advice without seeing any examples of your stuff, but:
- Learn shading - There are many styles of shading, some more or less detailed than others, so if you're not the type who likes over shading don't be fooled thinking that the solution is to avoid shading at all.
- Gain an understanding for how certain colors interact and blend together - This sort of overlaps with shading, as you can use different colors to shade or lighten something rather than using black or white, which preserves saturation of the colors and doesn't dull them out. Also known as "hue shifting."
- Use references - Even if you have an entire idea fully planned out, references can help to accurately translate them into your art. Additionally, if you like something about a skin someone made, download it and open it and try to study how they made it.
- Remember to texture from a 3D point of view - Minecraft skins have two layers, the body layer and the armor layer, making good use of the armor layer can give your skins much more shape and detail.
