I decided that, when two people don't have a strong preference, the best solution is for one of them to have a preference.
So for example, my wife might say, "What are you in the mood for for dinner?" which is almost guaranteed to start an "I don't care" bidding war. So, instead of fully committing I"ll toss a few options, and I'll try to rank preferences.
"We had chicken last night, so I'd prefer not that. Steaks sound good. Or maybe hamburgers. Do any of those sound good to you?"
And we can usually narrow it down that way. I've found that, sometimes, actually, I DO have a preference...and I wouldn't have bothered to check with myself otherwise.