Get a motorcycle and park it against the garage door. Jump off, and after the garage door opens, quickly press Triangle to get back on and press X to drive it in before the door closes again. With a car, keep pushing it towards the garage door until it opens, then keep pushing it until it is inside.
Park a vehicle already in your garage halfway out to keep the door up. Then, drive in as many other vehicles as you can fit. Make sure you can reach the doors on them or you may not be able to get them out.