This trick worked in GTA III as well. Drive up next to a hooker and wait. If you're driving a swank enough car, she'll start talking to you. Eventually, she'll get into the car. At this point, drive to a quiet spot and let the magic happen. The car will start rocking, and your "health" will grow from 100 to 125. Your pocketbook will take a hit though. This health-up only works until the next time you die.