I found the following advice in Daniel Dennett’s book Intuition Pumps and Other Tools for Thinking very effective:

How to compose a successful critical commentary:

You should attempt to re-express your target’s position so clearly, vividly, and fairly that your target says, “Thanks, I wish I’d thought of putting it that way.
You should list any points of agreement (especially if they are not matters of general or widespread agreement).
You should mention anything you have learned from your target.
Only then are you permitted to say so much as a word of rebuttal or criticism.

The goal is to get yourself and the other party into a frame of mind where each is open to the other’s viewpoint.