Yes, wheel chocks would help in this case. Also, use of the emergency brakes as well as leaving it in gear (EDIT- Leave it in gear if a manual or park if automatic). If wheel chocks are not available, you could use a largish stone which could be wedged between the tire and the ground on both sides of the tire. This would be on a tire which is not flat and multiple chocked tires should be utilized. Leave the flat tire on the ground while breaking the lug nuts loose and again while retorquing them. This all depends on the slope of surface in question, though. There will be a point where the slope will be too great to overcome with any of these approaches. In this case, you may have to limp your vehicle to a place where it can be set level. You could also try pointing the vehicle perpendicular to the slope, with the flat tire on the high side.
