I've been offered a job. How much should I counteroffer?

I've just been offered a job. The salary is more than I am currently making, and I'm interested in the job. But the offer isn't as much as I was hoping for. How much should I counteroffer without asking for too much? 10, 20, 30%? Any suggestions would be helpful. I'm considering asking for a car too instead of the mileage pay they offered.