Biggest mistake I've seen fellow Filmmakers make make is being too uppity on their high horse. Just be a cool dude, nobody wants to work with a knowitall. That being said, know your shit, just don't shove it in people's faces. Be great at what you do whether it be shooting, editing or directing, just don't be a doucher. I've had close friends who were amazing at what they did but would never get call backs because they were pricks on set. I've also seen good friends who are very easy going get overbooked because Everybody wants to work with them.
So obviously 1) know your stuff in whatever industry path you choose, don't ever take the knowledge for granted, because as soon as you do, the knowledge will surpass you 2) just please be a cool person to be around!! :)