In the regular game, particularly if you're already deep into it and don't want to start lifeline, stock up on box mines or regular mines (whistling mines are not recommended - after a few seconds, zombies ignore the sound and walk away). Get yourself a police car, and put more mines in the trunk.
Get in the police car, turn on the siren, and leave your fortified areas. Drive around areas dense with buildings and try to find a hoard. Once you find a hoard, lure it near other buildings that have not been cleared, to get even more zombies on you. Once you've done this, drive about 40 feet ahead of the crowd, jump out of the police car, and run another 40 feet or so, and drop a mine (make sure it isn't close to the police car, otherwise it will blow up your ride). Let the zombies get a little closer (taunt if you need to), and then run for it. The blast radius is pretty big, so you should be able to take out 8-12 zombies per mine this way. You can stock up on grenades to to clean up stragglers.
Once you've cleared a street out, jump back in the police car and do it again. Very easy.