The way I make massive money is selling foreign fruits. Foreign fruits, as you probably know, are fruits that are not 'active' in your town. Native fruits are fruit that you start out with in your town.
-Native fruits sell for 100
-Foreign fruits sell for 500 (So a full inventory would sell for 7,500 bells)

A trick I learn on how to get a foreign fruit is to send one of your villagers a letter. Nothing fancy, so you can send something like:

To [villager name]
How are you?
From [your name]

And make sure you sent a present to that person (it could be anything [I send seashells]) If your lucky, you could get a foreign fruit. Plant it anywhere. If you wanna save some space, plant your trees one space away from each other. (Sorry, this part is hard to explain...if I had a picture, I would show you)

When your trees grow, plant the three fruits. Continue until you feel like stopping. Then sell 'em!
Selling could take from 10-40 minutes, but it really depends on how many trees you grow.
~~~
Using this technique, I made about 3 mil bells =D

If I missed anything, can you tell me, please?
If I made a mistake, can you tell me, too??? =]