2014
For anyone else still playing New Leaf, the fastest way to earn bells is unlocking Tortimer Island. From there, cut all the tree, bushes and flowers in the middle and leave it barren. The only trees you'll want up are coconut/banana trees. Go at around 7pm-8pm (the reason you want to go this late is to avoid grasshoppers and locust that might still be around) and catch all the beetles (the rarer they are, the more you'll earn) and sharks can net you a lot of bells, too.
Fill up your island inventory to the brim and go back to your Town and sell everything you caught. You can make 1/2 a mill or more in a single day using this method.
Other tips to make money:
Sell only items like fossils or silver/gold ore at your retail for 9,999 bells (the max any item can be sold). You can sell any item worth 2500 and up at this price without your villagers second thinking the purchase. Or sell regular ores at 4? the price minus one bell (the max price they can be sold without your villager questioning the purchase). If you equip a silver shovel, you can find ores easier when you hit a rock for bells.
Hitting rocks for bells:
everyday you can earn up to 16,100 bells hitting a rock with your shovel. To maximize the amount of bells that a rock releases, dig three holes behind you, so that you won't move backwards from the recoil.
Another tip to make a lot of money is selling all your your perfect fruit in another town for maximum bells
and lastly buying turnip every sunday and waiting for the price in the retail shop to reach its peak. this is the worst way to make money out of the other ways (imo) since time traveling will rot your turnips.