Developing as a designer is never complete. There's no point where you can say "well, that's done". You're always learning, always adapting.
The sources for influence and evolving your craft are endless, with many coming from unconventional places.
The obvious ones are viewing the work of your peers, keeping up with trends, reading articles (well done, you can tick that off for today, now) and staying informed.
The world around you, however, can open incredible new ways of thinking about design.
Here Johan Ronsse even sites gaming UI, the incredible hardware of Iron Man and parts of his everyday life which inspire him, but it doesn't end there.
With everything you use, from analogue to digital, there are chances to think differently about design, to breathe new life into your work and ponder the question of 'where I can take this?'
Get inspired, question everything design, and evolve continually.
You grow as a designer through hard work, persistence, talking to your peers, and reading an awful lot. About a year ago I read a piece in the New York Times about people well into their 80s that continue to hone their craft. I feel like I'm only starting.