You'll need a decent amount of money (over 10,000); to do this quickly, set your characters on hourly jobs for a day or two. Alternatively, you can buy some money with donuts.
Next, you'll need to buy lots of the most expensive trees- I use the $595 trees. Place them in an open space until your money runs out and collect the experience from them. You'll get 70-100 XP each.
Now you need to sell ALL of them. You should get about 40-60% of your money back. Repeat the process and you should get a couple of thousand XP points. When you run out of money, just save up again.
Set all of your characters to their one-hour activities, hourly for as long as possible. It may be a little boring, but it works. Also, add a hundred friends on Origin (Google Tapped Out friends).
Tap their until you have collected from all of them; you should have got about 3000 XP and about $5000, which you can use to buy more trees and repeat the process.
If you have got some credit to spend on iTunes, you could buy some donuts, which you could use to speed up 24-hour activities, buy money or buy buildings that come with characters like my Frink's Lab.
Then you can set them on their one hour activity and repeat the tree process etcetera.
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