First, in my experience it helps to like other human beings. Barristers who fight shy of social contact with real people are hardly going to endear themselves to solicitors, or be able to cope with individual clients terribly well. That only leaves corporations (also staffed by real people - well some of them anyway) and prosecuting. A little restricting if you are just starting out. Even then, your solicitor may want to talk to you over a coffee and they will know if you are charm itself when there is work to be had, and bored and constantly texting (my daughters do this but I hope they will grow out of it) if not.
Second, it helps to be able to be part of a team. As a pupil or mini-pupil it will seem (and might well be) the case that your part is fairly insignifcant. But the art of working with other people, listening to them and being able to respond to their concerns is something you can watch for and learn, just as much as the art of persuading the Judge. Given that you have done 4 years law or less, and a lot more years basic human interaction than that, it may well be all you can offer at the beginning.
My late Head of Chambers, Brian Walsh QC was a fantastic example of someone who could bring everyone into a case and make them feel that they had a contribution to make - even the lay client. He took that ability with him when it came to Chamber's matters and went out of his way to explain his decisions and talk through people's concerns. That was an approach to which clients also responded which, given that Brian was famous for persuading people to plead guilty, meant that an awful lot of people ended up agreeing with him that it would be best if they admitted what they had done.
Third, don't be pompous. Respect and empathy are more and more important. The old days where the client looked up to you just because you were a barrister have gone - and not a moment too soon. Society is simply less deferential these days: the extent to which that has tipped over into complete incivility is a debate for another time, but clients of all stripes now expect you to treat them decently. Barristers who insist on running the case their way may be tolerated by solicitors but the approach is neither admired nor liked. A recent poll shows barristers believing that they spend enough time with clients, and clients believing the opposite.
Because the Bar is a small profession, people know each other quite well and are prone to gossip. However unbothered you are, it really doesn't help to be thought of as an inadequate human being. Treating other people appropriately will help. Otherwise, when you fall down, and you will, there will be lots of people waiting to say that you had it coming. The Bar is also a competitive profession. As a colleague of mine (now a Judge) used to say, "It's best to be good and to be nice. One can get away with being arrogant and unpleasant providing you're good. But X (name withheld) is crap and and arrogant and unpleasant, so everyone hates them." Don't be X.
Fourth, relying on being a barrister to make you feel important, or to help you feel clever or to help you feel thin is ultimately self-defeating. Either deal with those issues outside the job, or wait until they are sorted before you become a barrister. I passionately believe that this job is about the clients. Most of them know very little about the process to which they are subject and they are frightened. Barristers who do the job primarily because of what they get out of it are betraying the profession. However sad it is that someone has a self-esteem issue, it is wrong for that person to even risk preferring their own interests to those of the people they are retained to help. That is the antithesis of doing the job properly.
Fifthly, if you are someone who needs, or wants, notches on their bedpost, leave the urge at home. No one can control where Cupid's arrow strikes, but setting out to seduce your solicitors, your clients, your roomate or your pupil supervisor is best avoided. I am not saying that your eyes cannot meet your opponent's across a crowded court room - that has certainly happened. But that is different to a drunken encounter after a Chambers party with someone you already know has a partner. If your virtue is not intact, do try to make sure that your dignity is.
Sixthly, never crab your professional colleagues to opponents, solicitors or anyone else. It simply makes you look small and it will get back. Moaning that someone else has had a brief that you wanted, or a red bag when you should have one, or a pupillage, does not make your listener feel sorry for you in a good way. This sort of behaviour is a risk if you start to feel that you need that drink at the end of the day. It helps neither your practice nor your reputation to sit in bars, mouthing off about things that irritate you whilst drinking too much (whilst your significant other - should you have one - is presumably chucking the dinner away in disgust). If you need to drink or to moan then you have a problem. Work it through instead of taking it out on other people or the bottle.
Seventh, don't disguise what you are or where you came from. Although the Bar is a far more diverse profession than it once was, there is a certain tendency to behave, once one has arrived, as if one's background conformed with the public view. I don't think anyone now practising actively denies, for example, that they are Jewish but that was not always the case. If you are the first member of your family to go to university then be proud of it. It will not affect your prospects - any Chambers bothered about that is not a place where you want to be anyway, but I truly doubt that such places still exist. But pretending to be what you aren't, or striving for some type of social acceptance which matters to you, but no one else, is sad and simply contributes to the sterotypes of which we are trying to rid ourselves. Don't be part of the problem.
Eighth, don't pull up the drawbridge. Being worried about internal competition is a give-away to clerks and colleagues. Chambers these days are very much larger than they once were. If the best junior not in your set wants to join your set, only the weak and inadequate will complain. Don't be the person of whom it is apocryphally said that on the occasion of their dinner to mark 40 years at the Bar, the person proposing the toast said, "If X had had his way, none of you would be here tonight". On the same theme, don't kick down the ladders you have climbed up. I suspect this is self-evident. It is deeply unpleasant, and you may meet the people on your way down...
Ninth, try and do something about work/life balance. Have a hinterland. Taking work on holiday, or cancelling holidays may be macho but it's also stupid. You aren't making a statement about the size of your practice (which tend, in any event, to be the equivalent of that sportscar in terms of symbolism) . You are making a statement about your ability to handle your own life and the people in it. No one wants to come second to that massive case. You will work hard anyway, so have friends and do things with them. Try and have friends outside the law and certainly outside Chambers - don't sit around in an incestuous little group with all the perspective of a 2mm fixed lens. By the same token, don't sulk because you can't go skiing or sailing or walking 12 times a year.
Tenth, your professional integrity must be complete. The concept of justice is bigger than you. We can never guarantee to get to the truth (nor can advocates of an inquisitorial system). The truth is known only to those directly involved and to (if you believe in Him or Her) God. That is precisely why the rules are so important. If we cannot guarantee the right result, we must at least guarantee the best opportunity to reach it, and that the same opportunity is extended to everyone. To be a barrister is to be a proud part of that system. That is not just about showing your opponent authorities against them. It is about putting justice first, not taking a short cut to win, not keeping something up your sleeve.
Can you succeed if you don't follow these rules and don't have these qualities? Yes, (except, I believe, for the last) - but it is unlikely. But there is a wider issue. In the days when the job title itself brought respect, a barrister discharged their duty to their profession by not being dishonest. Nowadays, for the profession to retain the respect of the public we have to do a little more than that. I still believe (quaintly and in an old-fashioned way) that we have a corporate responsibility because an independent Bar helps the citizens of this country to have justice. If we let the profession down we ultimately let the clients down.
Finally, I am not saying that everyone can do all of this all of the time and I am certainly not saying that I have done all of this all of the time. But I still aspire to do so, and if I occasionally fall down I try and get back up. I am afraid, however, that if you think these things don't apply to you, there is not much prospect of you being successful and still less of you adding lustre to your profession.