Well... Please don't laugh, because this actually seems to work for me 9 times out of 10

I'm sure this is anything BUT the official method, but what I do is click the point from where I want to start the tracing, then deliberately head off in the WRONG direction along the coast. This has the tendency to make the trace go off in the opposite direction (as I'm sure you will already have experienced), which if you have deliberately gone in the wrong direction to start with just happens to be the right direction.

Then just back up and carry on as if you haven't just had to trick it into going in the right direction