Just read a fascinating article about decision fatigue (thanks to Joan Eisenstodt for the pointer!). A few things I learned from this article that may apply to meeting planning:
Participants tend to default to the easiest and/or safest response after having to make a lot of decisions: Put the hard questions and those that require thought at the top of the evaluation form so participants will be fresh to answer what they really feel.
Since people tend to shift to the default option after having to make a lot of decisions (the more complex the decision, the quicker fatigue sets in), break up your event decision-making sessions into reasonable chunks, with a lot of breaks to refresh your brain.
If your session have drained participants' willpower by making them make too many decisions or use their willpower too often, give them a quick hit of sugar before asking them to make any more decisions or avoid temptations (like skipping the final sessions to go sight-seeing or take a nap). Aha, so that's why we always get cookies at the afternoon breaks instead of something that's good for you...
Do you see any other possible learning points for planners in this one?