I use dilute all purpose cleaner (Surfex HD in my case) which I spray on to all surfaces in the car (in sections). Clean bucket of water to dampen a microfibre for wiping off. Gentle microfibre used for final drying wipe.

Alternatively use a quick detailer (e.g. Meguiars)

Not all microfibres are equal - some can finely scratch delicate surfaces and I've even had some that can mar paint lacquer. For piano black its worth investing in a better microfibre from a detailing supplier rather than any old one from a local shop (loads available - I don't have a particular recommendation). Cut-off labels if there are any and then fold the sewn edges into the centre of the cloth so that these do not contact the surface either.

Polishes can help the appearance of already scratched piano black as has been suggested. In most cases, this is because they contain fillers for this purpose. They're not actually removing the scratches. If this gives a sufficiently good finish it's the least aggressive/best option.