I am still new to this hobby but this is something I have done correctly in the past few builds...
When "panel lines" are available between the two different colored parts, it is better that you have the entire piece covered with tape. Then, use a toothpick to go along the "panel lines" (to increase the depth of the line for better guidance when using the knife). After that, cut and remove the parts that you want the paint to be on. This is exactly like doing window frames.
When the "panel lines" are not available, you have to use the overlapping tape method mentioned earlier.
Here's a pic that shows the result of the above process. Notice that the imperfection on the right seat is not a result of poor masking. It is a result of an accident after spaying dullcote.
Winston