That's a very interesting question, and all the replies were very interesting to read too! ( ^ u ^ )
I think constant practice is the key, drawing is my drug and I can't live a single day without it. By practice, I just don't mean drawing what I like all the time, I also mean trying to challenge myself into drawing something else. As a teen, I used to draw a lot of human faces but when I decided later on to add some more sophisticated objects, backgrounds etc... my art style evolved in a great way. I think it even changed a bit the way I was drawing human faces. It's hard to get out of our comfort zone but I think it's necessary to develop our art style.
Practice is also inseparable with observation. When I wanted to improve my way of drawing hands, I simply tried to reproduce what I saw in the real world. Like many of you, I also sought inspiration in other artists creations. My childhood was influenced with Disney animation movies and Franco-Belgian comics (I will forever love Hergé's, the author of Tintin, way of drawing hands). I only discovered the fantastic world of manga as a teen. I'm sure that playing video games also influenced my artstyle greatly.
And last but not least, I think the events in the real world has also a great influence on our artstyle. For instance, I had a phase of depression in my life and for some reason, my artstyle became more colorful and "cute". Some people tell me that my comics are light hearted and I assume it comes from the fact that I came across stressful periods in my life and tried to compensate it with my drawings.
So in a nutshell, I think an artstyle evolves with its author. It's a reflection of what we do (the more we practice the more it gets better), what we like (as we are influenced by the real world and other artist creations) and what we feel. I guess that the best way to develop an artstyle would be to push those three points and, as I mentioned earlier, challenge ourselves and get out of our our comfort zone 