Decision making and arguing are two very different tasks. For decision making I find that asking questions around the subject helps clarify what you're actually trying to achieve with the decision, gives you additional information to work with and a bit of time to think while they're answering.

Also a valid response to being asked to make a decision can be "I'll think about it and get back to you" (but always make sure you do get back to them about it)

The best advice I've got for most arguments is to not bother. If you've reached the point of arguing then egos are involved and people won't back down even if they realize that they're wrong.