How do professional mathematicians learn new things? How do professional mathematicians learn new things? How do they expand their comfort zone? By talking to colleagues?  
 A: It seems to me that the most important thing to learn when you're a graduate student is how to learn more mathematics. Everything else is detail. So you do what you learned to do as a graduate student (in order of increasing effectiveness, at least for me):


*

*Read papers and books (I'm actually unable to do this. I fall asleep.)

*Sit in on courses

*Work out problems in books

*Try to work out the details of a paper yourself and refer back to the paper when you get    stuck

*Set up working seminars with other people who want to learn the same thing


And I'll repeat what one of the other answers said: ask every dumb question that comes to mind and that you can't figure out the answer to. This can be done in person, by phone, or by email. Or even on MathOverflow.
A: Slowly and with difficulty, just like amateur mathematicians.
A: Well, understanding mathematics has different levels: Understanding is, mainly, pretending to understand. If you are able to cheat your professors, then your students, and your colleagues, it's OK. If you succeed to cheat yourself, then it means you went a bit too far. 
A: While we are all waiting for sensible replies let me say this: one can't learn a new area of mathematics without asking at least one hundred silly questions (which is why Mathoverflow is such a great website by the way).
On a more serious note: learning really new stuff involves rethinking the basics. Or, as some people would say, learning a new language. Almost everyone speaks some language, but learning a new one once one has learned one already can be tricky, and there are few people who can learn it as well as their first language, although learning to just communicate in a foreign language is not that hard. A possible analogy would be that almost any mathematician knows something about physics, but there are few mathematicians who have a really good command of it.
On the other hand, most people who are bilingual have learned two languages simultaneously. Moreover, they did it not necessarily because they are exceptionally bright at learning languages, but because e.g. one parent spoke one language and the other parent spoke the other one, or because the parents had to move from one country to another (a not at all uncommon thing among mathematicians).
So here is an obvious conclusion: one should try to learn as many conceptually different things (geometry, algebra, analysis, physics) as one can while one is still an undergraduate or a beginning graduate student, or in high school if possible. When one is an undergraduate, one can learn anything, no questions asked (except  for when the exam is); it can be harder later, when one is constantly trying to put things into perspective.
A: Try to solve a famous open problem in the new field. Even though you'll almost certainly fail, you'll learn a lot of new mathematics on the way.
A: Talking to colleagues is good. Also, attending talks, reading papers and books, and teaching - I never knew much about differential equations until my department made me teach it (I still don't know much about differential equations, but at least I know enough to do a decent job of teaching it). 
A: Teaching a course in something is the only way that I can really learn something new.
