I'll second some of the recommendations...

1. Masking tape on both sides of the joint where you don't want caulk.

2. Appropriate size hole in the nozzle at a 45 degree angle.

3. Nice smooth pressure on the trigger and even speed to apply a
consistent bead in the first place.

4. Plastic cafeteria spoon makes a nice finishing tool. I've also used
whatever other rounded impliment I could find that matched the radius I
was going for. The trick is, the bead should be super thin by the time
it gets to the tape edge. If not, you have to either use a smaller
radius finishing tool, or start over with the tape further away.

5. Once you get it close, stop. Peel the tape off and don't touch it
anymore.

Bobby