First, use quality masking tape. I personally prefer Tamiya tape over all others I've tried. It is more expensive than other general purpose masking tapes, but keep in mind that it is only really necessary for the edge. You can use regular tape to fill in areas.
Next, stick the tape to a hard surface. A piece of glass works well (take a picture off the wall and put it back before the family notices). I use 6" ceramic tiles I got from Lowes for about 0.50 each. Cut a thin (roughly 1/8" wide) strip of tape, but use a straightedge to get a nice smooth edge. Tape that narrow will follow nearly any curvature, and a single width of tape will make several strips. If you have to splice one piece over another make sure to stick the edge where the tape "Steps Down" very well or you will get a leak there.