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.