I used to eyeball a lot of anime drawings in junior high when I first started wanting to draw, and then gradually developed my style from there; then it kind of stagnated and I stopped drawing for awhile and my drawings really deteriorated over the next few years. I luckily was able to befriend some artists once I started university, though, and I think that really helped in terms of influencing my need to improve! The same thing happened on tumblr, and I kept repeating a process of seeing cool aspects I liked in other people's art and incorporating different things into my own style until it evolved into what it was today!
I think a key turning point in my improvement was figuring out the direction I wanted, and this happened a number of times-- my primary style is currently heavily influenced by Phobs and Len-yan while my secondary style is mostly influenced by Kali Ciesemier! Art idols, man.
I'm pretty sure a crazy amount of practice and experimenting helps, too; I didn't really improve much in the last half of 2012, but I drew something new almost every two days last month and forced myself to try new things, and as a result improved rather drastically!