Robert Rauschenberg once said:

I am trying to check my habits of seeing, to counter them for the sake of greater freshness. I am trying to be unfamiliar with what I am doing.

I've used a bunch of different euphemisms to describe this tendency; sticking your neck out or playing over your head are the two I come back to most. If you get comfortable with limits you've never really tested, you tend to not only stagnate and repeat yourself, but see other people's work in the same lights as well.

It's tough to talk about something like this without sounding like I'm recommending people do things that they simply don't like. I don't know that I would ever visit Poland on my own dime, for instance -- not when there's Japan beckoning to me for that kind of treatment.  But at the same time, I also know it helps not to be limited by your own interests whenever possible. Like Dean Sluyter says about doing the right thing, you lean whenever you can in the direction to go in without breaking your back.

It's also too easy to get dogmatic about what you think is the right way to expand your mind. Back in college I had a writing professor who suffered from this sort of thing. One of the students was having trouble coming up with story material that he felt was worthy of the effort, and as I listened in astonishment he grilled her about her life -- how old was she? had she ever traveled outside of the United States? etc. -- and came to the conclusion that she'd lived too sheltered a life to ever be a writer.

I was offended then, and I'm still offended now, even if I think there was a grain of truth to what he said. Yes, having some degree of life experience apart from the four walls of your room is a great idea, but surely there's a more diplomatic way to say it. Or maybe, in his fully-baked cynicism, he felt that if she was a real writer in her marrow, she would cuss him off and go write as she pleased -- a variety of reverse psychology that's about as insulting to everyone involved as it is naïve.

I've mentioned before that one of my tricks is to play the you-should-read-this-book game -- e.g., if someone's writing a fantasy story and they haven't yet read The Last Unicorn or Gormenghast (and all too often, they haven't read anything by either Peake or Beagle), then I send them in that direction. This, too, has its limits, but I can usually learn one thing from it if nothing else: if the person in question complains about being told to read something, then my guess is they're not going to be much of a writer no matter what I recommend.