The most common answer to this question is that you have to practice. You have to draw every day. That's true, that would help. But, I don't -- I don't have the hours or availability to draw hours every day. Yes, of course I only get better when I draw more. But, how do I make the best of my limited draw time?
On the other hand, find something that makes you produce art. For me, I have Trese Brothers deadlines weekly and I have to get pieces done. That makes sure, even with limited time, that I am drawing. If its a webcomic, a sharing site, a project you're setting your own schedule for -- find a reason to insure you are creating finished pieces on a weekly basis.
Back to the important question, how do I make the best of my limited draw time?
The Star Traders Banner Art - from 2011 and 2013
1. Look at other artists' work.
There are tons of great communities out there of very active artists -- such as DeviantArt.com or ConceptArt.com. Places where you can go and see the talented artists today show off their latest and greatest. Don't let this be disheartening. Yes, they are amazing -- but that's why you're training. Don't just drool, really look. Find some artists you like and follow them. How do they do it? How did they use shadows and backlight? What lines and techniques did they use to make that foreshortening work? How did they stylize his or her face? Really look, get down in the pixels. How did they do it? Soak that up and try it yourself.
Cyber Knight's First Character - 2011 to 2012
2. Use real pictures.
Another piece of classic artist advice is to get an anatomy book and to learn about the human body. It is truly critical to get this part right, otherwise you're subjects will always look off. If you look at the first Bodyguard Officer below, you'll see that classic flaw. Her breasts are impossibly high to her collar bone!
First, get a head start and browse the internet about "drawing human anatomy" and get a feel for the basics -- length of arms, proportions of the face and the like.
After getting the basics, one good way to soak up this knowledge without sitting by your anatomy book all the time is to use real pictures. Go on Flickr, Bing, Google and look for images of humans in the poses and stances you want. Look at real faces, real hands, real legs, and in the case of the bodyguard real breasts-and-collar-bone combinations! There are millions of photos out there, so use the resource to your advantage.
Star Traders Bodyguard Officer - Early to Late 2012
3. Read and watch video tutorials, speed draws.
Lots of great artists are posting screen-casts of their drawing sessions online, on YouTube or selling them for cheap. These are great resources. Try to follow them, try to make your own variations on their work. Watch them again, try again. You're building the knowledge base in your mind, and eventually your drawing hand will learn to follow! DeviantArt is great place for video tutorials (http://templar-assault.deviantart.com/art/Video-Tutorial-01-372790043) or you can go to a dedicated site like iDrawGirls.com for tutorials (http://idrawgirls.com/tutorials/). Or, YouTube if full of amazing speed paints (http://www.youtube.com/watch?feature=player_embedded&v=EbuFN71yUpI)