A clay bar is a detailer's best friend. For anyone new to detailing, a clay bar is a soap sized chunk of putty that plucks embedded contaminants out of your paint. Clay bars typically remove contaminants such as tar, bug remnants, railway fallout, brake dust and anything else invisibly coating your car's paintwork. You should Just make sure you use it before you wax (as it will probably strip that off too) and don't drop it on the floor as this will contaminate the clay.
Use a clay bar after washing, before a pre-wax cleanser, with plenty of clay bar lube and you will feel the difference. Compared to an area of paint that hasn't been clayed, the paint will be silky smooth. The Dodo Juice Basics of Bling Clay Bar has a gentle claying action and comes in a plastic storage case.