Talk over the phone first, straightening out any tensions that are still left between you two.
Agree to meet. Keep this part of the conversation short and upbeat.
Meet them in a low-stress environment or in a comfortable public place.
Talk as if you were talking to a friend, and try not to bring up any memories of your relationship that could cause nostalgia or even pain.
Be as natural as possible when meeting with your ex and try to act as if they are a really good friend. Don't show them you care too much or they might mistake it as a sign that you want to get back together.