Check your ego at the door - You are a good photographer or you wouldn't be on the job in the first place. No one is going to hire someone who does lousy work. They also will not hire someone AGAIN who thinks they are a gift to the photographic community. No time for drama. You have a job to do. You are there, do the job and leave your ego at the door. It's not needed at the photo shoot.
Lend a hand - What? You are too good to help carry equipment, move lights, check you own gear. Since when? But you say, "When I was coming up I had to schlep all the gear, why should I do it now?" Because it's your shoot. Your job. And you hated it when you were an assistant. A major plus is that you can look at each piece of gear again to make sure everything is there. Besides, you will not believe how grateful the people working for you will be to see the boss pitching in. They'll do more for you voluntarily than you could ever get out of them if you just stood by watching everyone setting everything up. Don't get in the way of the trades, but find something to help with. You will never regret it.
Be on time - You don't get to be a prima donna on a shoot. Show up just a little early and stay just a little later. It builds team spirit. And some of the really fun stuff happens when the shoot is over.
Want to be asked back? - No one is going to ask you to another shoot if they all have to walk on egg shells around you. Yes, a photo shoot can be stressful, but you acting up is not going to make it less so and you won't be coming back if all you do is add to that stress. Be nice. Relax.
Help others learning the trade - You haven't always known everything. You had to learn. Take the chance to help up and comers get along in the business. I've trained hundreds of people to be boat captains over the years, (my previous occupation) and have never lost a job to someone I have trained. I'm a good captain with years of experience. A hard combo to beat. And as an added bonus, I've had captains come to me and tell me about jobs they thought I could do better because of my years on the water. Be the same as a photographer. Be a mentor and help. You'll be amazed at what you can learn from people you've helped.
Make sure your equipment works - Carry spares if you can. If you can't afford whole other rigs, at least carry extra batteries, memory cards and flashes or have a plan to shoot around equipment that goes bad.
Don't blame others - Things happen. Stuff goes wrong. Don't blame your crew, your Mom, your dog or your goldfish. Ultimately it is you who represents the company. If some one working for you makes a mistake, you take the blame. The crew should never be blamed to the client. The client probably won't believe you and you'll look even worse. Take the blame, fix the problem, get on with the shoot. Then, don't fire the person, other than you, that caused the problem. Believe me, you're not perfect and the person responsible will work twice as hard to prevent anything from happening the next time. If it is your fault, blame the cat.