I want to stand up for myself more because it seems that, over time, my friends begin to order me around, ignoring what I want to do and expecting me to be willing to follow their every whim, but getting angry if I stick to my guns on something I want.

So here's an example. I'm at Walmart with my friend, and he keeps saying he wants me to go get a coffee at the McDonald's. (HE doesn't want coffee mind you, he just wants to sit there, so he expects ME to buy some.) I tell him repeatedly that I already have a coffee in my hand and that he can get one if he wants, but I don't want to go. He brings it up over and over and actually starts snapping and saying "'mickey's! Come! Let's go! Mickey's!"

What's the appropriate way to handle that without "starting something" and possibly losing a friend over it?