Whenever I'm at a restaurant, or ordering food for whatever reason, I will check the menu for an item that prominently contains mushrooms. If such an item exists (and it doesn't sound awful or ridiculously expensive), then that's what I'm getting. Decision made.
I started doing this mainly because I like mushrooms, enough that it felt like a really easy way to hand off decision making. It's also good in that, unlike the above, I don't get just the exact same thing every time (although mushroom burgers are starting to seem pretty same-y).
Also reduces the odds that you'll be looking at someone else's plate thinking "I wish I'd have ordered that instead" :)