For masking over rivets, molded seams, etc. on our plastic shells, use a thin, narrow vinyl striping masking product to create the edge. Then cover the rest with blue painters' tape.
For plain, straight lines on flat surfaces, Tamiya masking tape works well.
And if you are feeling particularly obsessive with creating a straight edge on a flat surface, apply the tape on a piece of glass and create your own "edge" with a sharp razor blade and steel rule; then remove the tape from the glass and apply it to the model.
EDIT: Saw the thread's title was directed to "Painting Experts..." which I am not; merely passing along what I've been taught by experts and used more than a few times when I've masked and painted LIONEL model trains.
