I get the marks off mine with a certain brand of liquid silver polish I found in the U.K supermarkets. It is 'Carr & Day & Martin' silver polish.
It was the only one that did not carry warnings about using it on Nickel.
I just put a tiny bit on a soft duster and rub in, then polish off straight away with a completely seperate part of the duster.
It has not scratched or degraded the nickel at all - and works every time. I do clean the marks off the same day though, so there has never been much chance for the acids on my skin to really act on the metal.
I will of course stand corrected if Tim has other recommendations .
F.F.