Let's face it - things don't always go the way we want.  It is important to keep your wits about you.  Don't lose your cool!  Step back an analyze the situation.  Try to figure out if it is something you are or are not doing or if there is some external element affecting the outcome.   It could be as simple as your voice inflection or body position.  It could be your dog isn't feeling well or a foreign smell in the air!  Is this review for your dog or are you trying to do something new?  If a new concept, have you properly introduced the concept by "teaching" before you have expectations for how your dog will train for it?

Remember:  If you can't figure it out, sometimes it is best to move on to something else or just simply call it a day.