One of the best guides you'll find is on Associated Content. I won't post the link (so as not to be considered SPAM), however if you go to Associated Content and type "Zombie Farm guide", it will pop up. It covers FAQ, tips and tricks, how to earn free brains and gold quickly, the time cheat so you can invade more often (and get brains), what the deal is with the 'lost' mausoleum key, and even a fix to a common invasion crash problem (it isn't foolproof, therefore doesn't work for everyone, but it's worked for quite a few people so it's definitely worth a try). A section of the article is below, but please visit AC to read the rest as it is 5 pages worth of information that took me quite some time to figure out, compile, organize, and write. Hope some (if not all) the information helps! :)
"If you've found this article, chances are you are as addicted to the ever-so-popular Zombie Farm game as I. I write this article to serve as a comprehensive guide about everything Zombie Farm! Here are the events that led up to my amazing find - hopefully this doesn't happen to you, but you can surely benefit from the tips below regardless.
Recently, Zombie Farm came out with an update that caused the app to crash multiple times during my game play. As per an Apple representative's instruction, I deleted the app and reinstalled, not knowing that it would delete my entire farm and all progress I had made. I was on level 20 when this happened, had spent money on brains via in-app purchases, and had upgraded my zombie farm to the largest size available. What I realized (and what most players don't know) is that the game is saved in a file on your phone - not on a server - therefore once you delete the app, the save file will also be deleted.
So there I was, starting over with the dreaded six plots and a fresh new Zombie Farm. I was quite upset considering the time and money I had invested, so I began trying to figure out ways to earn brains, gold, and experience points to regain what I had lost in the shortest time possible."