This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From: email@example.com (Perry Hunter)
Try scotch tape instead of masking tape. It should release fractionally
faster and >might< reduce scorching of the lower stage.
In some cases, it's possible to line the inside of the top of the lower
stage with 20lb xerox paper, and it will take the damage rather than
the exposed section of the stage. It's not possible to cover everything
(slip fit couplers , etc. prevent lining all of it) but it can help.