@Dinh: I'll only comment on the newer stuff, since it's more relevant: 

1.) Never paint with the colors black or white. [i]Ever.[/i] (Especially for skin tones.) They invariably suck the life out of all other colors around them. A tutor of mine once told me something that kind of blew my mind once it clicked -- the colors black and white don't actually exist in real life because they're always either absorbing or reflecting the colors that are around them, which always adds a tint to them. Painting with black and white to mix your shadows and highlights always has a very plastic/artificial look because the eye knows it's wrong. (The brain may not have the knowledge to explain why it's wrong, but your eye will still know something looks off. Kind of like when anatomy is off -- you may not be able to explain or even correct it because you don't have the knowledge to do so, but even the most inexperienced amateur will be able to look at something and know that it's not sitting quite right.)

Look at shadows around you when you're outside next time -- you'll notice they're not black, usually just darker shades of blue (depending on the time of day). The more you look around and study the lighting around you, you'll find it to be more and more true that black and white are never actually black or white like you think they are. Knowing things like this will help your color work immensely and instantly bring a level of realism into your work that you didn't have before. An excellent book for learning how colors and lighting temperatures (warm and cool lighting) work is [url=http://www.barnesandnoble.com/w/color-and-light-james-gurney/1100185195?ean=9780740797712]Color And Light by James Gurney[/url]. This thing is practically my bible. It's written for traditional artist, but everything he says applies to digital art, too. It's a fairly inexpensive book and it's super helpful. Also, painting objects from life will help you to get the hang of colors/lighting better. Doing still life work will teach you almost everything at the same time (you learn lighting, color, texture, perspective, proportion, how to control your brushwork, etc.), so they're really the best way to learn to paint. 

2.) Anatomy -- yours is having some issues. For instance, the ribcage is too short, but the torso (abdomen, from under the breasts down) is too long (based off the measurements of the woman's head). Anatomy is the bane of every artist and it'll drive you crazy... I don't think anyone every truly masters it, they just gain enough knowledge to get by. Even the most seasoned artists continually do anatomy studies because the human body is something that is so complicated, even the slightest movement will entirely change how a muscle is rendered or the perspective of something. The only thing you can do is study to fix it -- try to copy photos and measure out the proportions from the figures as closely as you can. Check out the Andrew Loomis books ([url=http://illustrationage.files.wordpress.com/2013/04/andrew-loomis-figure-drawing-for-all-its-worth.pdf]especially this one[/url]) I linked under the reference section in the first post and you'll find some excellent books to help you do anatomy studies. Loomis is great for getting the basics down in an easy to understand lessons. (The wording is uncomplicated and he takes things step by step, not assuming anything on the student's end.) George Bridgman is my favorite for learning anatomy because he makes it look so effortless, but his books can be a bit difficult to understand. (They're really technical, which is excellent when you feel more comfortable drawing the figure/knowing the basics. He can be a bit overwhelming if you're still trying to get a handle on things, however, so I usually recommend him for intermediate level (+) figure artists.) Definitely check his work out, though -- his line work is really admirable, as is the way he handles his figures.

That all said, I can definitely see potential in your work. I'll be very interested to see more from you in the future!

@ Allen: Perspective, like anatomy, is a very tricky subject. There's an excellent video by an artist called Carl Dobsky on the subject, where he really takes and breaks down how perspective works and ways to correctly calculate all your angles/distances with minimal fuss. It can be tricky to find (because I don't know if it's even available through legal means anymore), but it was called 'Linear Perspective with Carl Dobsky'. The tutorial had two parts -- one for one point perspective, the second was for two point (or more) perspective. You can see the trailer for part one [url=https://www.youtube.com/watch?v=kZyRKKoN0kk]here[/url] and part two [url=https://www.youtube.com/watch?v=5HFOO-Lrct0]here[/url]. I bought this back a few years ago when it was first released, but I can't seem to find it to purchase from the original source anymore. (It's just not available on their site.) I'm not usually an advocate for 'alternative' methods of acquiring media (*ahem*), but it may be your only option in this case. Otherwise, I'd suggest just doing a search of youtube videos and seeing what you pull up. (I know there are a ton of them on there.)

And, in regards to your question about work looking flat, you haven't really added in any lighting to your drawings (at least that the ones you've posted here) so far, so of course they're going to feel a bit flat. Adding in lighting/shadows turns your flat lines into rendered shapes. If I draw a circle on a paper in just lines, then it's just a circle. If I add shading to that circle correctly, it will then become a sphere. (Even though they're both drawn from the same exact circle, adding in the lighting/shadows ultimate gives it form and depth.) Try adding some shading into your next drawing and see how it looks. ^_^

@Sheep: That's actually a pretty cool drawing. I actually like the stylized look and the muted colors. Nothing really I can think of to critique, but I'd definitely be interested in seeing some more work from you!

@LowKey: What is that a fan art of? (So that I can look it up and make suggestions where appropriate.) Meanwhile, try to use more confident lines when drawing -- you have a lot of scratchy lines clumped together to form your edges, which makes it look like you're very unsure of where to make your marks. If you use a single line, then your work will look far more confident. The plus side of working digitally is that you can 'undo' your lines if they comes out wrong, no matter how many times they do. Take advantage of that and try to use single strokes to define your edges, rather than several scratchy ones.