Here are a couple of tips I think should help anyone to improve their art.
Be critical of your art
Look at your art and look for things that you personally feel could have been done better. Take those thoughts into your next piece. Remember that there is always place for improvement, no matter how good of a artist you are.
Be prepared for critique
When submitting art to a site like deviantART then you should be prepared to receive critique by other users. Take every constructive criticism into consideration for your next piece.
Premium Members have the awesome Critique feature available. Remember to use this feature as it can be very helpful!
You have no idea how much helping others, helps you. When you comment on a piece, give them tips on how it could improve. It's easier to find faults in other peoples work rather than yours. So while writing the comment you might notice that you do some of the same mistakes.
Learn from others
Looks at how other people do it. Observe what makes their art so good. Learn from them. Rather than mimicking them, you should incorporate what you just learned in your own style.
If you only make the same things over and over again then you might not improve that much. When you start experimenting, trying different techniques and styles, then your will start to learn a lot more. Take risks. Sometimes you will get negative feedback but those will only help you. If something seems too hard to do, then you should do it.
Never give up
Don't be discouraged by all the amazing artists out there. Don't think that you'll never be that good. Even they started with stick figures. You should be inspired by them, not discouraged.
I hope some of you found this article to be helpful, maybe even inspiring. If that is the case, then my mission is accomplished.
NOW GO MAKE SOME AMAZING ART!