No real strategy here, just make sure you're always stealing parked cars.
If you need to grind this out at the end of the game, load up your inventory with melee weapons and / or bricks and stones, then run circles around the car dealership. Break into each and every car, then immediately jump out and boost the next one.
By the time you've done a full circle, the first load of vehicles should have been replaced with fresh, locked ones. You can find several mallets and crowbars as you circle around, too.