Norway is a good country to hitch in, of course depending on location! Hitching in big cities is almost impossible, unless it's weekend. People have a tendency to always be in a rush on weekdays. The main thing to be aware of is the rapidly shifting weather, so be prepared for anything, as they say in Norway: "there is nothing called bad weather, only bad clothing".
Places close to small towns are good hitching spots. But do take care of your appearance, as people are a bit judging in this country.
You should definitely write your destination (if you know it) on a big piece of paper, with a thick marker. Don't necessarily write your final destination, but the next town or city in your desired direction is a good start, once you're in the car you can find out if it's going any further.
Also, you can contact truck drivers and companies to hitch a ride if you're traveling far.
Most people speak English well enough for reasonable communication.
Don't try hitchhiking out of Oslo's airport.