I read a article recently [0], and I think you may like reading it.
From my username you can see what I'll post here is really naive. But what works for me is failing. Failing sets a precise goal of where is my limit (be it in knowledge, or something else). So keep trying, keep failing and soon you'll find you keep improving. Other than that, a good mentor would help a lot, don't be afraid of asking questions when you don't understand something or think the person can help you improve.
[0] - http://www.daedtech.com/how-developers-stop-learning-rise-of...