Zombietoaster says:
Draw alot. Try a new style of drawing or shading for each new piece. Try to emulate different peoples styles of shading and such. Combine different styles into other styles.
Look at stuff you like and try to figure out what it is that you like about it.
Practice anatomy. If youre gonna draw humans then you gotta know anatomy. And dont do it by looking at other peoples drawings. Everyone, even the best of the best, have some anatomy errors, and then when someone learns from those pictures, they will have the same anatomy errors, but worse.

So look at photos.
Look up some good stock in the recources section here on DA for example.
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Kousagi-Kun says:
~drawing:
1. Like the others said, anatomy is important. study up on your proportions.

2. Working at different angles is a good idea. Remember perspective as well. For help on perspective, I usually go up to a mirror and do the pose im drawing, to get an idea of what it should look like.

3. Details can also make the drawing better. Things like folds, designs, accessories, etc. Like with the perspective thing, look at your own clothes for some ref on folds and such.

4. Something I ALWAYS have a hard time doing is lightening up the pressure on a pencil. Try to avoid pushing too hard because like zepaul said, it'll be harder to erase.

5. Shading can also draw attention to your work. A basic thing to remember is to define your lightsource. Usually you would start with your darkest shades around the sides that appose the lightsource. Next you add in shading to the details and such, and a softer shade near the darker ones. Account for the areas where the lightsource is being blocked by another object, like a hand covering a shirt or whatever.

6. I'm not sure if this will help you, but you could try drawing the most complicated parts of the body individually, at different angles. That way you save time from doing the whole body, and you have the practice for when you actually do draw it.


~spriting:

You can also use the drawing tips for spriting.

1. Colors are a big thing. Usually there's 2-3 colors per part of the body. Try not to use too many. Also, make sure the colors don't overlycontrast or arent contrasted enough. basically make sure the colors stand apart from each other to some point, but aren't a drastic ways apart.

2. DO NOT HAVE JAGGED EDGES ON THE OUTLINE. It really ruins the quality of the sprite.

3. The outline of a sprite is just as important as shading, in my opinion. Make sure to make it good. With making the outline, take the same steps to making it you would for drawing.

4. Okay really important, make a copy of the outline before you start shading. In case you mess up and all.
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I just posted this as a guidline that will help me improve my skill level each time I log in DA. >.< I'm very forgetful.