I didn't come up with it, but I made it through 114 flags with this:
Source (not my video):
"Zombonis" in the outside lanes seem to blow up just before running over the first plant, the plants in the pool take care of the inside two lanes. If there's a zamboni right behind the lead one and not on top of it, you may have to help destroy it before it squashes something however. Also watch out for jack-in-the-box zombies that sometimes survive long enough to detonate in the lanes adjacent to the pool.
I pretty much just saved the Cob-Cannons for the big guys, replacing pumpkins as needed due to dolphin riders, and football zombies that manage to make it up to the plants along the top and bottom of the screen.
The one other thing is to absolutely plaster the lawn with sunflowers at the beginning in order to maximize your sun while the zombies are still pretty weak.