We have a system. It's a good system as long as I dont need that many groceries.
First of all, it helps if you have a good baby. (Have I mentioned that Layla is damn near angelic? Oh, I have? Well let me say it again. She's awesome.)
This is because if your baby is crying in the grocery store, people will look at you like you must be pinching her. They will whisper in asides to one another. "That baby isn't happy. Why doesn't she do something?" and other such nonsense. They don't care that the child is neither too warm nor too cold, is dry, has already rejected a fresh bottle and 3 varieties of binky, and simply wants to go home. But you can't go home, because there's no food there, and this is where they keep the food.
So. Having a good baby is definitely a plus.
First things first I find a parking spot near the cart corral, so I can just snag one right there instead of carrying the car seat thru the lot. Why don't I use a stroller, you ask? Have you ever tried to push a stroller and a shopping cart at the same time? It's ridiculous.
The baby's carrier goes into the basket of the shopping cart. This is why my system is best if you only need so many groceries. I can't put the carrier on top of the seat thing, because although I'm fairly certain it would be safe, I'm less than 5 feet tall and I can't see her if she's way up there.
I shop the way my mom always shopped. It's force of habit. Through the produce, all the way around the outer edge of the store, then up and down the aisles. Items are strategically placed around the baby seat in such a way that they probably wont topple over onto her, but if they do for some reason, she wont get hurt or cold. Milk and ice cream actually goes under the head of the carrier, because they're big and cold and it just makes the most sense.
The tricky part is when we get home. I came up with a system that works pretty much like a relay race with a relay team of me. Luckily I live on the first floor.
Park in front of building. Baby into apartment. Leave her in the car seat in the living room.
Then, run the groceries from the car into the vestibule. Run back and lock the car. Open the security door and move all the groceries to the other side. Let the door shut, unlock apartment door. Move all the groceries from the hallway into the inside doorway of the apartment. Shut the door so the cat doesn't escape. Then move all the groceries from the doorway to the kitchen. Put anything that might melt or spoil away immediately.
Unhook the baby, change her, and give her a toy to lick (she's really into licking things right now.) Then put away the rest of the groceries while simultaneously warming up a welcome home snack baba for the baby and eating something yourself quick before you pass out because you didn't eat before you went to the store. Mostly because going to the store is such a pain that you waited until there was nothing in the house but seven week old oranges and a box of Uncle Ben's.
Some women do this with multiple small children. My mom, for example. I can't for the life of me figure out how.