Many people have one line that they use in a loop to control the blocks and the traveler. Some people use two different lines (possibly of different colors) and then tie them together so that they form a loop. This seems to reduce the confusion of which line to pull when.

In Rick White's book "Catamaran Sailing For The 90's" he says that the traveler is used to control the lower part of the main sail tension and that the main sheet is used to control the upper part of the sail. I think that this means that the main sheet tightens the boom downward which flattens the sail and pulls the main tighter higher up the sail, thus flattening the top part of the sail. Is this the right idea?