Advice from a veteran:
Don't forget to bring a large bag or granny cart to put your stuff in (the IKEA shopping bags are a great idea).
And bring a checkbook, as payment is either check or cash. No credit cards. Have more than one check, as you may find yourself going back for more.
I always come armed with a list and prioritize it. My kids are long and skinny and hard to fit in pants. I started shopping with a tape measure. Now I don't bring home dresses and pants that are too short. Also, I do less impulse buying and stay on track for the things we need or want.
I also carry a bottle of water and lipstuff and leave my coat in the car. No matter how cold it is outside, I don't want to deal with another bulky garment. I always get warm and a bit dehydrated. For those who are PG, I think the water is important, as well as a little portable snack.