1. Be a bitch
When I started working, my immediate reaction was to be nice and helpful. Contrary to what my instincts say, being helpful does two things. It gets you more work and it earns you a reputation for being a pushover. In the worst cases, being helpful makes you spend money that you were not obligated to spend and will not be repaid for. Learn to say no and do nothing without paper. There are exceptions to this rule--be helpful to people within your company (namely, your boss) and your foreman (because they will make your life hell if you don't).
2. Ask all questions
Problems come up. That's the nature of the beast. Before you run to buy a new fixture or send a panicked email, stop and ask questions. What happened? Did you see it? Is this on the drawings? Whose mistake is it? Did we take pictures? All of these questions drive to one singular point: You must understand the situation completely before you can propose the best solution. The first solution you think of may be quickest, but in general, it is always the most expensive. Slow down, because the person you go to with the problem is going to ask the same questions that you should. Take time to find the answers first.
3. Hedge your bets
If it's not on paper it doesn't exist. We have to commit to two things: schedules and prices. It is very tempting to be helpful (see point one) and give a lower price or an earlier date based on one week of hunky-dory work. Optimistic predictions become lies when the crap hits the fan. When giving things in writing, you need a disclaimer. "This is based on a three day turnaround on submitted documents." "This is assuming no delays in shipping." "This date is riding the concrete being poured." "This price doesn't include overtime." Things will go wrong. It is your responsibility to foresee them and cover your butt ahead of time.