Hey man, cool work indeed, you seem to be improving. Slunko is right about learning perspective, it's really important to practice perspective in my opinion. Just learn and practice on paper the basics: you must know that the horizon line is in fact the level of your eye, the vanishing point is situated on that line, you can have one vanishing point like this:
http://avatar.pure34.com/users/blizzard/1point.jpgIt's pretty simple, everything will tend to that point, only the verticals stay aren't affected.
The 2 vanishing point perspective is actualy not much harder to understand, instead of having one point on you horizon line you will have 2 points, which will give you this kind of effect:
http://a33.idata.over-blog.com/450x338/0/25/19/69/concepts-plastiques/Dober-Town-Final.jpg you usualy see these 2 vanishing points on street corners, 2 vainishing points still DONT affect your vertical lines.
Well the 3 vanishing point is a bit trickier, but not that hard to understand. It's almost the same thing than the 2 points, but this time there will be a 3rd point, which will be situated upper or lower the horizon line: if you put your 3rd point above the horizon line, it will create what is called a Low-angle shot, on the contrary if you put it below the horizon line it will create a High-angle shot. It's great to create a dramatic feel.
http://b7.img.v4.skyrock.com/b75/copyleft/pics/1311045720.gifI insist on this because it's a MUST to know that, if you want to work on video games or in the entertainment field you have to know this. And there not really any tutorial to become good at drawing/painting, you have to work really hard and practice a lot and one day it will pay, there's no secret. Just practice EVERYDAY!! Same with anatomy and design you might have guessed already.. it requieres time and passion, but in my opinion it's worth it, it's something that keeps you entertained, you have to deal with it, be curious and look everywhere, try to understand how light works, and notice than light is less satured than shadow when the light is specular. Well, open your mind and your eyes, be curious, always keep a sketchbook on you and take notes by drawing (i think the notion on taking notes is actualy funnier than pure drawing, but it's just me) and move on do it, enjoy doing it and practice understand practice pratice take a coffee practice observe practice.