Non-car guy here. In the rare circumstances I need a ride somewhere, or need use of a car, I either rent one or ask my friends to do me a favor. If I ask them to do me a favor, I make it clear there's no expectation AND I'd pay for gas / compensate them for their time. Some decline the cash, and do it for free. Others let me use my credit card to fill up their tank. It depends. If I'm visiting a friend out in the suburbs, they usually offer to drive me to the Metro. I'd never expect them to drive me home.

I would say that the best think you can do is be honest with your friends - if they ask too much, or you feel they're taking advantage of you, tell them. It's one thing to help a friend out, but if you think they're asking too much, be clear. Don't do what one person said and lie to them about why you can't help them out, just say something. And don't feel like you *have* to help them - set expectations and be firm. Let them know that it's not always convenient to drive long trips in the city.