Don't get anything acidic. P21S gets a great review around here but I haven't tried it. Usually I just use a different wash mitt and car soap but it doesn't get it 100%.
Was: 2008 335xi sedan 6MT Black Sapphire Metallic
Is: 2013 Fiat 500 Abarth Nero (black metallic)
Wife's: 2013 Infiniti G37 S sedan 6MT, Black