Generally, if I'm under a car, there are jack stands and, if the wheel is off, that is slid under the frame along with a couple boards(2x8s) to reduce any clearance. If the wheels are on, I have lots of 2x6s, 8s or 10s which go under the tires. Additionally, the jack stays in place if at all possible. Also, the wheels are chocked. My ramps - after the umteenth time they went skittering away while trying to drive up them, they went to the metals dumpster at the land fill and have now been 'reborn' as Toyota front fenders