Trying to get straight, clean lines when caulking can be an exercise in frustration if you're out of caulking practice. With painter's tape, however, it could be a piece of cake every time.
The photo above from the Swing N Cocoa blog shows how easy it is. Just apply the painter's tape above and below the area you'll be caulking, apply the caulk, smooth it out, and remove the tape while the caulk is still wet. Voila! A seamless caulk job.
(Caulk and painter's tape come in handy together, by the way, also when you're painting the edge of a wall.)