Just to share my experiences trying to hitchhike the ferry to Newhaven...
I'd heard before about hitching cars so I tried waiting infront of the passport control. Someone picked me up but as they'd made a reservation, the control kicked me out when they saw I wasn't on the ticket! Then they tried to charge me 65 euros to cross as a foot passenger! After I kicked up a fuss they brought it down to 50, still outrageous but after failing to get a lift to Calais (very difficult route) I had to pay it.
I think if you go into the office with whoever books, it is possible to get in a car for free.
Good luck vagabonds!