Did mine last year, was amazed how much crud came off what I though was quite clean paintwork :o
Firstly washed, then rinsed & dried, then clay bar with Meguiars quick clay, rinse again then Autoglym SRP,
then Autoglym EGP to seal it, then Collinite 476S wax to finish :D
The clay bar will get dirty Chris, just kneed it until it becomes clean again and carry on :)
Replace eventually if it becomes exessively dirty, or bin it straight away if you drop it on the ground.