Etiquette question: when should you offer (or ask for) gas money?

What’s your personal policy on asking for or offering gas money when giving or getting a lift? Do you think that when you agree to give someone a ride, you agree to absorb the cost of gas, regardless of any other factors? Or is there some distance at which it becomes obvious that some gas money should be exchanged? What if it’s a short distance, but traveled frequently? Or does it depend on your own financial position, your relationship with the ride sharer, or something else?