I'm on the other end, I work for a large company and we often hire contractors.
1) Get your work orders in writing, every time, every change.
2) Tell them exactly what you are going to do before you do it (requirements reviews, design reviews, schedule reviews, etc). It's better to give them too much information about what you're doing than not enough. Be a pest, they'll love it.
3) Quality first. If you can't get it done on time, tell them as soon as you suspect there is a risk, they'll understand. Don't rush it and deliver substandard work.