This is an alteration of another solution and is very effective as it allows you to change/re-randomise the next level all of the way through the mode. So you can avoid expensive levels that are full of Munchers, potato bombs and squashes, or just generally hard levels
Creating checkpoints brief guide;;
Allow game to 'run under lock' can be changed in the options menu
1) At the start of the stage, hit the power button on your phone, this will lock it and turn it off.
2) Now turn it back on and unlock the phone. It will ask you to hit resume (this has just created a save point).
3) If you mess up, waste the rest of your sunlight in a lane thats empty. DO NOT hit retry, but the main menu button that's in the top right.
4) Navigate back to I, Zombie endless and it will ask you if you want to continue, select yes.
5) VERY IMPORTANT: as soon as you continue the game you will need to repeat step 1 to create another save. If you don't and then fail you will go back to level one.
-Selecting and playing other modes will wipe the checkpoint.
For a more detailed, easy to understand guide on these steps see Haxa Fox's solution; Solution for Better Off Dead in Plants vs. Zombies (WP)
A checkpoint can be made at any point in a level and as many times as needed, doing it at the end of a level means that if the next level is hard you simply go back to the checkpoint and repeat the process until you're happy with the level given
Note: I recommend you have your phone on charge or have high battery as if this runs out it will create a checkpoint wherever it runs out at (I got messed up this way)
This is worth a second mention as it's so easy to forget in the monotony of it all, every time you reload a game you have to go through the process again to create another checkpoint
I usually made the end of level checkpoint when my final zombie was about midway down the path to give me plenty of time to do this multiple times if needed, also the money bags that you collect at the end of a level can be left to create more time to repeat the process (I got 5 repeats from the money bags but I'm not sure how long it actually lasts)
When you are happy with a level a checkpoint can be used to keep it for multiple retries
Feel free to comment any suggestions