toddler's defiance: how to respond?
I think the most important thing is stay calm yourself. If you yell or get agitated, the power struggle is almost guaranteed to escalate. Then explain that from this point your expect her compliance. Be very clear so that can be no doubt about what you mean.
I like to use humor, but not sarcasm. I think a little humor can help defuse a situation. The trick is to turn the joke on yourself, not on your child. Defiant kids are often hypersensitive and can't handle a joke aimed at themselves, but can laugh about something else.