Your best friend
If you are hitchhiking in Chile, the Ruta 5 is going to be your best friend. You can go all the way from the south to the north without big troubles. Most likely a truck driver will pick you up, as they are often driving alone and are bored. Many truck drivers go from Punta Arenas to Santiago de Chile or north to Antofagasta or Arica.
South of Santiago de Chile is incomparable more traffic. Since the south is pretty busy on the road, 200 km north of Santiago there are very few cars. Maybe one in 1 or 2 minutes. Of course, depending on season, daytime, etc.
Best spots are next to the toll stations, since cars are slow (or standing) and will have time to have a closer look on you. Don't hesitate to stand right next to the road, Bernhard did it very often that way. Cops won't bother, don't be afraid.
I think there are many nice spots where you can pitch a tent a bit hidden and off the road. But be aware in the dessert to hide a bit and make sure you have at least 4 litres of drinking water with you. Bernhard talked to other hitchhiker on that road who got seriously in trouble because of this fact.