Don't use a spong mate, use a mitt like the Meg's Lambswool mitt. The fibres on it are long which meand they pull the dirt off of the car and up the fibres, rather than a sponge which can trap the dirt between it and your paint, then when you move the sponge around you get lovely fine scratches!
Also make sure you take good care of your washing equipment, if you drop any of the microfibres or your mitt make sure you give then a thourough cleaning to remove any grit etc before puting them anywhere near your paintwork.
Cheers.