Erm... Tips:
- Use layers
- Your lighting's inconsistant, draw a 'lamp' in your page as a reference so that you don't get mixed up.
- Don't use flat black as a base colour.
- Do the shading on a separate la
yer on multiply and the highlights on another on soft light (if you're using PS) - Save it as a .png file
- Use references to help get a better idea on the anatomy for it.
- Use the smudge tool in 'circular' type patterns, not harsh flicks.
If you're confused on what some of these are go here. Remember, this is just how I do it, you can completely ignore my advice completely and still be amazing. So go ahead and develop your own ways of using whatever software you're using.
I actually find it kind of funny that you didn't do any shading. Most people do shading without highlights, but it seems that you're the complete opposite xD
Also, if you really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really really want to improve dramatically, you should practice everyday. It doesn't matter if you do a 5 hour painting or a 5 minute sketch. Just draw something xD