For example, compare a typical flamewar comment to this:
> In practice, open source and free software are nearly equivalent, but the open source people reject our philosophy. They think it’s just fine if someone wants to write proprietary extensions to their software. So we have to differ with them.
Also, it's good to see surface issues (e.g. practicality, compatibility, etc.) reframed in terms of the Free Software philosophy. For example:
> Your question implies that software is good, even if it takes our freedom away. I don’t accept that.
Making distinctions like this can probably save an awful lot of arguing, by rephrasing things from "clearly XYZ" into "even if you don't agree with ABC, can you see that it is a valid position to take, and that XYZ makes sense in that case?" (where, for example, "XYZ" might be "Flash should be disabled").
For example, I'm often asked how I can possibly live without Facebook/Photoshop/whatever. A useful answer is that humans have survived for thousands of years without them, and I'm no different. By controlling the context of the question, something strange can seem normal or sensible.
Thankfully there is a spectrum of software available, so other people's choices and preferences (whether proprietary or not) aren't forced on everyone.