I try to be the sort of shopper you'd like to be behind in a queue: I have my card(s) ready in a neck-purse or in my pocket, and I kind of multi-task by interrupting my packing to put my card in the machine, pack a bit more, retrieve card, etc. If I want to rearrange my trolley or pack more neatly, I pay first, then pull aside to sort it out.
I know it's only a little thing, but people in a queue are sometimes stressed and rushed, and it just seems selfish to me when certain customers take great care to get everything back in the trolley, neatly packed and sorted, before beginning to hunt for their purse and find their card to pay. Even worse is when it's a couple, and neither of them is ready to pay until they've both sorted the packing to their satisfaction!
Most supermarkets will provide a member of staff to help pack if you are struggling. I did this a few times when I had back problems, and also used their help loading the car.
Only this morning, I asked if the man behind me would like to go in front of me as he only had one item. He shrugged and said he wasn't in a hurry, but it's always worth asking because sometimes people have a bus to catch or children to collect, and a few minutes can mean a lot.