Credits to pwnthis.net
I was just feeling bored and tried his method on the 3rd game that they offered and needless to say, it works for My City Life too.. so here goes, become lvl 80 and a millionaire easily.

You need:

Charles 3.4.1
Firefox
Flash 10

Step to Step guide:

1) Open Charles
2) Play My City Life
3) In Charles, find for the url ‘games.mmo2.myfarmvillage.com’
4) Expand > Expand ‘City Life’ > Expand Definitions
5) You should see ‘Buildings…’, ‘Decorations….’, ‘Terrain…’
6) Right click Decorations and select Breakpoints
7) Refresh My City Life. The game will freeze at the loading and a Breakpoint tab will appear on Charles
Click Execute
9) ‘Edit response’ tab now appears. Click that.
10) Click HTML
11) First item on the list is ‘Road’. Change the value of ‘Sell’ and ‘plantXP’ to 999999
12) Click back on the Structure tab, and right click Decorations and select Breakpoints
13) Click back on the Breakpoints tab and select Execute
14) Now the game will continue loading… Buy a road and watch your exp rise. Sell it and you will gain the amount of coins you input in step 11.
15) ¡Enjoy!