I second what's been said so far. I have to admit though, that I don't always follow my own advice. I really should.
Here is what I would add...
Along with the drawing suggestions that have been included here, I am a strong believer in making time to view the works of other accomplished artists. More specifically, artists who create work that you are personally drawn to. At first, you may not quite understand why certain pieces jump out at you. But after a while, things will start to become a little more transparent. Maybe it's the looseness or tightness. Or the use of a limited color palette. Or a wide color palette. And when these discoveries start happening, I think you'll see improvements in your own work. Surrounding yourself with good art is key.I find it similar to playing a sport. Let's take basketball for example. If you play with people several levels more advanced than you, after a while you can't help but improve.