Brass bathroom sink- How can I clean it?

I would just clean the sink with plain white toothpaste(NOT GEL).The toothpaste will aid in keeping the sink clean and shiny.

I agree, toothpaste works copper, brass, silver, gold. You can also try ketchup. Slather it on, let is sit for several minutes, wipe and rinse.

Liquid car wax.

Maas metal polish is the best. If the brass has been lacquered, the lacquer will need to be removed with something like Citristrip, first. After polishing, then you'll have to decide whether to lacquer it again, or polish it periodically.

I agree it's important to first find out if there is a lacquer or other coating on the brass.