I would say the best way is to get your self a clay bar kit.

If you do not want to order online i think you can get a kit from Halfords for about £20 that comes with everything needed and instuctions on how to use.

Infact just done a search on there site and they have there own or a Meguiars kit
http://www.halfords.com/webapp/wcs/s...0151&langId=-1

Myself personally i would spend the extra and get the Megs kit.

Just make sure you wash the car first and make sure the panel is well lubricated when using the clay bar
Most clay bars you will be able to split down into 3 or more pieces so you can do a few cars but should you drop the piece you are using discard it as it will pick up dirt you cannot see and will mar the paint

If you want to buy online a good cheap one would be the Bilt Hamber clay http://www.bilthamber.com/autoclaybuy.php the best bit about this clay is you use water as the lube so no need to buy or use expensive quick detailers or dedicated clay lubes.

I have never used the above but have seen some good reviews on it.

Just a few options
Hope this helps