How do I make a good mask for a traveling matte?
 

I just finished a small personal project in which I wanted to use a traveling matte so that I could key a moving boat over a title. You can see it here:

YouTube - An Afternoon in Hong Kong

To create the mask, I exported a frame from the 1080i HDV source in Premiere Pro CS4 to Photoshop, played around with its gamma, then converted it to gray-scale. I manually removed the background, in some places having to simply erase it by hand. I re-imported it back into Premiere and used it as a traveling matte to key out everything but the boat.

It worked, more or less, but it is not very clean.

Whenever I see "making of" videos of films involving a lot of CGI, they always shows these absolute perfect masks. How in the world do they make them? For the boat mask, there were many fine lines that didn't reproduce will in the mask, and it was really difficult distinguishing between low contrast areas in the exported still.

Is there a trick to doing this, or is it simply a matter of having enough patience to go pixel by pixel?