Actually the only answer to feature creep regardless of project size is "we'll get back to you with a new quote". Simply because you dont know how much effort is involved on the spot.
People who buy things from you know how to screw things out of you as a rule and this is a well known tactic usually disguised as ignorance.
I've been handling offering flexibility in fixed big projects by sharing my Pivotal Tracker story board w/ feature estimates and letting them choose which features to axe to get their new priority scheduled.
In addition I've also been offering a fixed amount of "freebie points" for 'not yet discovered features'. That helps define that yeah, I'm going to do a couple extra things not defined in the scope, but it's fixed amount and when you've used them up, there's none left.
People who buy things from you know how to screw things out of you as a rule and this is a well known tactic usually disguised as ignorance.