Agree. Two bucket method using a wool wash mit is your best bet.
If the paintwork is in good condition then get some good wax on there, Auto Glym HD wax is great, also Collinite and similar products will give the same effect. It will look better, stay clean for longer, and make it easier to clean.
Might be worth finding out how much it would cost to get someone to machine polish it and get rid of all the swirls and scratches first before you seal the paintwork with the wax.
If it's black, you will find it easier to get a good finish if you do it in a cool shady spot.
I've had black cars before - hard work but they look awesome when they are clean.