For hitchhikers Argentina can be divided into two parts. The north, which is pretty okay to hitchhike, and the south, which is a complete disaster, especially compared to its neighbour Chile.
map24 is in Portuguese but has information about the whole of South America. A normal map is pretty expensive, but there is a road map containing many countries of South America for something like 10 pesos.
In Argentina it´s normal to ask the people filling tanks at gas stations to ask people for you!
If you´re a foreigner, do mention it! As a Dutch guy it seems to be a lot easier to get rides.
Number plates are white, composed by three white letters and three white numbers, so if you´re in some other country and hitchhing to Argentina, look for these plates!
There are quite some peajes, or toll passages. In the south however, these are not so good, and you will just be sent away after a while.