When I used to do checkouts, how the customer was treated depended very much on how they treated me. If they were rude and awkward, then I'd work as fast I possibly could. The nice ones always got help.
Now I'm on the other side of the counter, I'm deliberately awkward if the checkout staff are in a mood. If they try to rush me, I deliberately take my time and don't take out my purse to pay them until I've packed my shopping. If they tut at you, smile sweetly and say 'just a moment, dear' (the 'dear' is vital in the patronisation process).
Something else they don't like - you know when they give you your change, and they put the notes and the receipt in your hand first and then place the coins on top, so everything slides out onto the counter whilst you're fiddling with your purse. Try to keep a hand free and whip away the notes and receipt before they can do the coins. They hate it.
Then make a mental note of the assistant for next time and, when you pay them in cash, do it back - notes first, then coins on top before they've had a chance to think about it.
And finally, try to arrange a mock row with your partner over the checkout, the more 'threatening' the better. Nothing more certain to put the wind up them than the fear of being caught in the crossfire.