Most people usually arrive on Salt Spring by way of one of three ferry routes, landing at Fulford Harbour, Vesuvius, or Long Harbour. At Fulford Harbour is a difficult place to hitch since there isn't any area for cars to pull off along the hill leading away from the terminal. Your best chance is to thumb standing just outside Patterson's parking lot if there is room there for cars to pull in. If you're heading to Ganges and can't get a ride with the wave of ferry cars, either take the bus (meets most ferries) or walk to the head of the harbour and stick your thumb out by the Fulford Inn.
Hitching out of Ganges back to Fulford, Embe Bakery on the edge of town is a well known spot to hitch. Many cars won't be going all the way to Fulford. However, you ask if they are going as far as Blackburn, since there is a nice pull-off for cars there.