Comment by SkyMarshal

A simple rule can help with this - rephrase all your assertions as questions. Questioning instead of asserting is inherently less argumentative, is less threatening and more likely to cause the other side to seriously consider your point instead of reacting defensively automatically. It also signals that you respect their opinion, making them more likely to respect yours. You accomplish almost all the same goals as asserting, but in a gentler, more mutually respectful and ultimately more effective way.