When challenging someone's perspective stop and ask yourself these questions...

1) What do I understand about the others perspective?
2) What do I understand about my own perspective?
3) Could I be wrong?
4) What do I want the other to understand about my perspective?
5) What do I want the other to understand about their own perspective?

If you find yourself skipping the first three, you're not really trying to have a discussion and may want to check your ego.