One of the most stressful and perplexing parts of modeling is preparing for a go-see. A go-see is basically a modeling interview. The casting director wants to see models in their natural state so there is not much one can do to prepare (fortunately or unfortunately).
After years of modeling and going on countless go-sees and castings I have my routine down. First of all, as a model it is your responsibility to stay in your consistent shape. Whether that's size 2, 4,....or 16 please don't fluctuate. Keep handy a "modeling bag" so you don't have to scramble when you get called for a last minute go-see. Backpacks are ideal since they keep your body balanced.
I like to keep ready plenty of comp cards, my simple black modeling pumps, my nude modeling bra without padding (companies don't like padding), a black Gap tank top, black cotton American Apparel leggings, and a fresh set of nude underwear. Only wear these items for modeling so they look fresh. Additionally, a tape measure, hair elastic, safety pins, tissues, and maybe some face powder. It's best to wear minimal make up.
Remember to smile, be polite, and friendly! Think objectively and don't take rejection personally. It's pretty simple in all actuality.