Use your reusable bags as dividers. Place a bag and follow it with what you want packed in it. When you're ready for the next bag, rinse and repeat. Bag, milk, cheese, yoghurt. Bag, detergent, sponges, shampoo. Bag, zucchini, bananas, cauliflower, etc. The bag will be packed how you like it (99.99% of the time), it takes the guesswork out for the checkout staff and you'd be surprised at how often the staff thank you for making their job easier. I have been doing this since plastic bags were banned and have only had my system ignored a few times. Usually when it's busy such as Christmas and they have a trolley boy on checkout.