Maximize your Sunflower plant power!
Just like the first Plants vs. Zombies, Sunflowers are mostly important. You want to aim for Two rows of Sunflowers on each level, so they can support all the rest of your planning . Combine them with Potato Mines to cover your Sunflower planting, and you're pretty much set!
When supplies are limited, get the most bang for your plant bucks
On levels where money or plants are limited, place plants that have area-of-effect attacks - Threepeater, Snapdragons, etc. - in lanes 2 and 4 and they'll cover the whole lawn for you.
Use food on a plant to get an running attack
You can use food to make a plant that's still in the middle of its refresh time "wake up" and attack quickly . This is really useful for Coconut Cannons.
Note: This trick won't keep a plant "awake", it'll go back to its refreshing sleep immediately after its attack is over.
Kill the app to reshuffle the Purple World deck
Don't like the cards you're dealt in the Purple World level? No problem. Just hit the Home button to exit Plants vs. Zombies 2, then double-click the Home button to open multitasking, then either touch and hold the Plants vs. Zombies 2 icon until it jiggles and tap the X to kill it (iOS 6), or grab the Plants vs. Zombies 2 card and toss it away (iOS 7). Once that's done, re-launch Plants vs. Zombies 2, and you'll have a whole new deck of cards to choose from!
Use Bonk Choy and Spikeweeds to survive Pirate World
If you're having trouble surviving Last Stand III in the Pirate World, try this formation. Using only Bonk Choy and Spikeweeds, you'll be able to complete the level without even looking. The Spikeweeds don't get eaten so the damage the zombies without ever having to be replaced. Any zombies that make it to the end get boxed into oblivion by the Bonk Choys.