Buckets. A seemingly often overlooked tool of the trade. As with any tool, they need to be looked after well. These get rinsed and cleaned after every wash.
I use 3 buckets, each containing a grit guard, with every wash. One for shampoo, one for clean rinse water and a third for wheels.
This helps to stop the risk of cross contamination and minimises the possibility of creating swirls in your cars delicate clearcoat.
Next time you use or pass your local cheap quick wash, have a look and see what they use. I can almost guarantee it will be one bucket with a sponge that's used for multiple cars. Maybe a second for wheels if you're lucky.