Sorry, I don't have much time right now to write a detailed answer (I'll be around later to) but I want to stress this:
Consistency in tonality.
Today we have no clue what shot will be edited against what other shot and I know from personal experience mixing that consistency in production sound saves hours of time for the dialogue editors/mixers.
By this I mean 2 people having a conversation with each other that you know will be edited together back and forth but may be shot hours apart to have a similar relative distance from the boom/lav. You don't want someone with proximity effect and someone far away from the mic.
Do your homework. Look to see what may be needed for the shoot and bring it - like the answer Edward gave. You never know what will come up or what you may be asked for.
I personally never back down or compromise quality by one inch. If you consider that what your microphones are picking up is 90% of what you will be putting out the theater speakers, this is a correct estimation of importance to production sound. Do not let yourself be "bullied" around on set. Fight for your quality of sound. If you don't, it's like the Director of Photography saying "Well, the lights really couldn't get too close tot the subject today so we're going to have to re-shoot the scenes we filmed today". Carefully analyzing Avatar and The Hurt Locker led me to believe that the sound oscars went to The Hurt Locker because the production sound was so phenomenal (they only ADRed 4 lines or something on the principal characters).
In my experience in doing ADR, unless you are working with a brilliant actor, the original production sound will ALWAYS be the better delivery on the actor's part. It will also be better sync, match the projection, match the emotion the actor has on his face, BECAUSE IT'S THE ORIGINAL. Pardon me but I think the fact you have to do ADR on 98% of a movie is a criticism to the production sound team (unless there are weather or ambient sounds you have no control over like planes, ocean, rain, etc.) But, doing ADR for an indoor talky scene on a sound stage is absolutely doable to keep the production sound.
Get wild lines from the actors when they're standing around - coordinate with the director ahead of time. Get some wild lines that are not even part of the script that you think you might need, and effort grunts and such, so you have them in a library for that actor. I know that The Hurt Locker used many lines and pieces of words from other takes that weren't used in the movie. This is for example, the actor cussing because he's frustrated, or efforts doing something physical, etc.
Grow eyes in the back of your head: The best boom operator I have ever met (said to be the best in the business) walked backwards while keeping his eyes on his mic and actor while stepping over cables and generators and equipment while walking backwards - if he tripped he would have ruined the scene.
Stay alert. Especially when you're shooting on film. Film is extremely expensive when you have to re-take things. It's a lot more expensive than hard-drive space or DVDs when you're shooting on HD.
Work out your arms - especially doing incline benching. Buy a light mic. The Schoeps CMIT 5U weighs possibly less than a 4th of the weight of a standard SM57. It actually weighs about as heavy as an expensive ball-point pen. That can help your arm fatigue enormously. If you don't believe me on the importance of working out and getting your arms in shape, try painting your ceiling with a roller, or simply just hold your hands and arms up above your head - IT'S TIRING!!
Become inventive. Mics are small and can be hidden other places than on a boom pole. Put it inside a plant. Put it inside someone's baseball cap. Cleverly work out how to mic someone in a bikini. Always check out a scene before-hand if you have to time and work out the best position of the mic.
Now more than ever with multi-camera shooting it's most important to coordinate with the cameramen and director on exactly where you can be and still be out of frame because of the cross-framing that happens. Sometimes there can be 4 cameras going at once and you need to know exactly where they will be to be able to stay out of frame.
EDIT: I forgot to mention an important point, and that is you must have breath mints at all times. If you're micing up someone you're going to be very close to them. Always have breath mints or gum handy in your pocket when you go to strap a lapel on someone you're going to get really close to.