said by Beezel:
Also using tape on both sides of the line before caulking helps.
That is what I do. I put tape on both sides leaving about 1/8"-3/16" gap on either side. Caulk the corner and then use my finger to smooth it out a couple times. I then quickly pull the tape off and run a clean finger down the caulk line lightly one last time and that gets rid of the tape edge making a nice smooth line.