Put your hand in a plastic bag and rub it on your paint if it gets held up and you feel stuff then you need clay. It won't always get dirty but after you use the paint will be super slick.

I don't use the stuff that comes wtih the Mothers kit. I'll use clay magic... it's blue and comes in a tub. I use a spray bottle with soapy water to lube the surface. Generally I'll wash my car but not dry it. Then I'll clay it using the soapy water in the spray bottle to further lube the surface. Then you "shuttle" the bar over the surface of the cars only in straight lines, not circles! Go front to back on the top surfaces of the car and up and down on the vertical surfaces. Keeping the surface well lubed is key.