The one way to increase your overall art in any digital media is to return to paper and get traditional art down. If your lineart is messed up it's probably due to lack of anatomical knowledge/making what you think it should look like rather than what it does look like. If you're not doing well with inanimate objects, draw/paint some still lifes (lives?). If you're having trouble with organic things get some anatomy books. I've got Encyclopedia Anatomicae, and it has helped immensely.
That being said, when you pixel for accuracy and realism, try to have a reference for the object you're making, either at your desk or in a picture. If you're making something unrealistic, try to have a basic knowledge of the object you're caracturizing or the material it's made of.
Zoning in even more to the question you actually asked, try making lineart then coloring it in 4color greyscale. That way, you improve not only your lineart, but your coloring as well. I mean, sure the actual choice of colors is important, but you can work on that after you get down the ability to make apparent forms with just 4 colors.
