I will tell you what we usually do in this instance is a little less drastic than a piece of trim. Assuming you are referring to an interior corner, we will caulk this corner, then paint into it.

The method we use usually covers the less than square results of the sheetrock finisher.

First caulk the corner with an acrylic/latex caulk and allow to dry over night. The best line will be produced by leaving a "fat" caulk line. In other words, allow the bottom of your finger tip to smooth the caulk not the end of your finger. This will be most helpful in the final stage.

Second, paint the lighter of the two colors all the way through the corner, do not stop at the line as this will require you to paint two straight lines as opposed to one. Be sure to apply two coats and allow to dry.

Now comes the secret. Get a pencil with a fairly blunt, not sharp, point on it. Run the pencil lead down the center line of the corner. Now you have what should be a decently straight line to follow for the darker corner. Don't worry if the line of first coat of the second color is not as straight as you think you can make it. That is what the second coat is for.