My two pals, AD and TOTWTYTR both have excellent posts up that offer sound advice to new medics. And, to be honest, I can't believe AD remembered our conversation all those years ago in Austin. I'm honored.
Frankly, there's little more I could add, but I'm going to anyway.
The Good Lord gave you two ears and one mouth in that ratio for a reason.
Spend more time listening than talking.
Listen to your patients. When possible, ask them a question that can't be answered with a "Yes" or "No." Get them talking. Then listen to what they have to say. A wise Physician Assistant I once worked with told me "The patient's history and physical exam will give you the diagnosis most of the time. Fancy tests and x-rays and such just confirm it." That's a truism I've kept in my pocket over the past decade as I've worked in EMS.
Listen to the doctors. They're better educated than you, and underneath the skin of every physician is a frustrated teacher yearning to breathe free. If you get a patient that doesn't quite meet the "text book picture" of a disease or injury -- and trust me, most of your patients will fail to meet the text book picture -- get the doctor talking, and listen. You'll learn something.
Listen to the nurses. Like the NCO in the military, it's the nurse that really keeps things running in the hospital. A wise physician will listen to the nurse, and if the doctor is smart enough to do that, you should too. Nurses have a different outlook on patient care, and broadening your view will help you become a better paramedic.
No single text book contains all the knowledge you need to do your job.
I really wish those public service announcements on TV that implore parents to read to their kids would expand to include paramedic students. Read! Get your paws on every book you can find about medicine, and read. Dr. Bryan Bledsoe has a 5 volume set of texts that cover a veritable cornucopia of information about prehospital care, and even those don't cover it all. Read! Get yourself a copy of Tintinalli's or Rosen's guide to emergency medicine. Then read it. When you come to a term you don't understand, go look it up. Have a copy of Taber's on the back of the toilet tank. Find a library that has various journals -- Annals of Emergency Medicine, Prehospital Emergency Care, Journal of Trauma, Heart -- and sit down to read.
Read! Read a diverse selection. I have texts on cardiology, medical-surgical nursing, critical care paramedicine, air medical transport, wilderness and remote medicine, orthopedic trauma, renal disease, critical care nursing, pediatrics, and internal medicine. I've given away twice that many to other folks.
You always have time to think.
Despite what the television drama shows or paramedic instructors tell you, there are very few times where you must act immediately! Damn few. I can think of two calls in 10+ years where I had less than 30 seconds to make a decision, and one of those was an easy decision to make. When things don't make sense, and you're not sure what the right course of action is, your best bet is to stop, step back, and think. Obviously, some of the basic stuff is a good idea, and comforting measures go farther than you think, but overall, you should engage the gray matter 'tween your ears and think.
One of my pet peeves, and the answer I give when people ask me in interviews "What's your weakest area?" is that I have absolutely no tolerance or patience for people who don't think. You've been entrusted with the care of another person's life and health. That's not a burden to take lightly, and it requires that you think. If you stop and think, and your thinking is awry, I'm OK with that, and most experienced and quality paramedics are, too. We don't mind if you make a mistake in your thinking. That's easy to fix: Sit down with some coffee, talk it over, and find out where your thought process was flawed. Simple, takes about 45 minutes. But if you tell me "I wasn't thinking," then I don't have any use for you, and would rather you sit your rump in the front seat and stay out of my way.
When it doesn't make sense, stop and think.
It's never a bad idea to put the patient in the ambulance and go to the hospital.
Along with the thinking bit above, don't forget that your job, ultimately, is to take the patient to the big building where the doctors and nurses work. You're not curing disease out there... you're helping people feel better in the short term, providing them some peace of mind, and applying temporary care until you get them to the hospital. If you can think of nothing else to do, put the patient in the ambulance and go to the hospital. This was the first thing my paramedic instructor told us on the first day of class: "If you remember nothing else after the next 14 months, remember that it's never a bad idea to take the sick person to the Emergency Room."