1. Being Social w/ Other Composers and Musicians
There is a lot to be said about building a community of composers where you live and learn. Usually, this is somewhat built-in due to the fact that you all go to the same school and are in the same classes. But, being social with your colleagues outside of an academic setting is incredibly important. As a general rule, composers are particularly anti-social creatures. We sit in front of our computers/piano/whatever for hours upon hours engaging in a pretty selfish enterprise; pouring our ideas and feelings into a complex and complicated language that mostly only we understand. At least at the end of the day, musicians go to orchestra rehearsal and play in chamber groups. We don't even have that. So, getting out of our own headspace and socializing with each other is important. Building/strengthening a new music community is good for everyone. It promotes a supportive environment. You can even count socializing as professional development for your future self (if you need to rationalize going to grab a drink with your "friends"). This is something you are going to have to do as a professor/teacher/whateverelseyoucandowithacompositiondegree and it is important that you are good at it.
Also, you never know who is going to help your career down the road. The more positive connections you make, the more possibilities are out there for you down the road. This is really a side-effect and not a primary goal. You shouldn't be fishing for stuff when hanging out with people. That often leaves a bad taste in people's mouths about you.
I was recently at a festival and I was talking to a bunch of performers. One said that they were giving a recital of new music and asked if I knew any pieces for X instrumentation. Now, I have one. The stupid thing to do was to just blurt ou, "OHMYGOD I TOTALLY HAVE A PIECE FOR THAT AND YOU SHOULD TOTALLY PLAY IT." It comes on a little strong. Instead, I gave them a few other pieces which I loved and could talk about and slyly work into the conversation that they had really influenced my piece in this genre. The performer instantly wanted to talk about my piece and it ended up being performed (along with the two pieces I recommended as well).
2. Stop Talking Shop in Mixed Company
We can too quickly get wrapped up in our own little bubble and alienate other people because we are always "talking shop." I have heard the flip-side from my wife a million times and have experienced it with some of her business contacts - its so BORING to not know what anyone else is talking about. Try to include the "non-members" in the conversation with questions (just like dating someone). OR - have other interests that you can talk about. In a world where specialists are dying out, its probably not a good idea to only focus on one aspect of life (especially music). Anymore, the job market doesn't demand the "master" but rather the "adaptable student." I think this is a life requirement as well.
3. Develop Soft Skills
All of the above so far falls under this category. I cannot tell you how awkward some composers are. So horribly painful to speak with. By soft skills, I mean knowing how to converse effectively with potential students, potential performers, or (even more important) potential employers.
When I came out of undergrad with a MUED degree and wanted to be a composer, I was looked on with a little disdain by some of the institutions to which I applied. Dan Asia at the University of Arizona, called me on the phone and spoke to me for about 45 minutes which just started out with, "Hey, I received your application packet and I just wanted to get your story." So I talked to him and essentially talked my way into a master's degree. Up until that point, composition was a hobby for me.
I cannot stress how important interviewing is. When you apply to grad school or to jobs, they get hundreds of applications that pretty much all look the same. Looking good on paper gets you in the door. Interviewing well in person gets you the job. My point with all this is that I think we are always so focused on putting out content that developing some of these life skills gets swept under the rug and forgotten about until it is too late.
4. Have Opinions (but share them tactfully)
First of all, the worst thing in music is indifference. It is a symptom or not caring or not listening. If you don't care, what are you doing in music? Go to business school, make money, and play videogames on nights and weekends. It is easy to be indifferent because you don't have to explain yourself. You don't have to back up your arguments. You don't have to listen and can still shake the composer's hand at the end and say, "nice piece."
Flip-side. If you don't like someone's work, maybe wait to talk to them in private and have a conversation. A composer friend recently told me about a mutual acquaintance who came up to him after his piece was premiered and shook his hand and said, "Not your best work, but ok." He's lucky he didn't get punched in the face.
This one always gets me. You are paying to come to a school with some of the really great composers in the country and then you complain about them and disregard their advice, citing that they don't really know the piece or don't share your style or whatever...
...who the hell are you?
The best kind of student acts like a sponge. Take everything in. Try things. Make mistakes. Later, decide what is relevant to you. How many famous composer's do you see that still claim pieces from their student years? No one. You aren't going to write any masterpieces right now. So why not try out what your teacher is saying and see what comes out of it. There are things I've learned from private lessons (as well as just from my peers) that I think about every single time I sit down to write. There is also a ton of stuff I've since disregarded or forgotten. But, you can't forget what you didn't hear in the first place.