surf the net to find images of sketches with the consrtuction lines still unerased. also find quick rough sketches of artists you like. you can learn a lot how they approach every stuff they draw. what do they do first? how they mass out an object? etc...you can learn more looking at these kind of sketches than tight or finished stuff. i recommend you google frank cho or kevin nowlan because they have a lot of unfinished stuff on the net.

you're work is cool as it is my friend. i dont have a hard time imagining your work hanging on a wall in a fine art gallery.

anyway i always keep 3 things in mind when i draw:

1.)make a solid looking subject:meaning good silhoutte, good pose and stance, solid anatomy if it is called for.
2.)use blacking well. realism is all about contrast
3.)if there seems to be something off in your work chances are that something IS wrong. this goes for perspective too. find it, correct it. don't be afraid to use an eraser.
4.)be patient. you wont correct something that you are used to over night. in time your goals, i assure you will be realized.

...OK,4 things in mind not 3 ...hehe!