If you are poor or just want more money, here are some ideas for you.
Clean out your closet
Are you sometimes just too lazy to go to sell something at Nook's, so you just put it in your closet? Well, you can make a decent amount of bells from there. It actually depends on what you have in the closet. With a closet that's not very full with some common things, you can get about 700 bells. But if you have a full closet (or close to it) with a variety of items in it, then you can earn about 30k. Don't go selling special items that you can't order back.
Don't buy everything
Do you just buy items to fill your catalog? Don't. It's a bad habit and you can lose a lot of money, even if you sell it back. You only get 25% of what you bought it for. (e.g. If you buy a moon for 32k and sell it back, you'll get 8k.) If you see something that will go nicely in your house, get it. Remember, just because you can afford it doesn't mean you can afford it. Say you have 2,000,000 bells and you buy a Royal Crown. You now have 800,000. Decent amount, but now you don't even have enough to pay the final mortgage.
Paying off mortgage
Don't save money to buy from Nook. Use it on the mortgage so that when it's finally paid off, you can make money and never worry about mortgage. It also saves money to never pay off the last mortgage when you have a mansion. What's the point? Also, when you pay off all your mortgage for another expansion, don't time travel to the next day. Spend your time trying to make as much money as you can.
Money trees and rocks
If you plant money with a golden shovel, you can sometimes get a money tree. The more you plant, the better chance you will have. Note that the tree will only grow 90,000 bells (3 bags of 30,000), so only plant 30,000 bells and below. Also remember that once you shake a money tree, the money will not grow back; the tree will become a regular tree. Don't try using a large amount of money unless you have enough to make up for it. Don't try this trick until you have paid off your mortgage entirely, or you are about to pay off your mortgage that day. Every day, there is one "money rock" in town, sometimes if a neighbor is near a money rock and you talk to them, they may say something about a money rock nearby. Find it, and hit the rock with a shovel or an axe and money will shoot out, you can get a total of 8,500 bells if you hit the rock 7 times. To hit it 7 times easily, just dig three holes (see below) behind any rock so that if it is a money rock, you won't have to move forward when you bounce back. There is only one money rock all day, every day.
Hole formation for money rock
You can get bells for fruit, but it depends on if the fruit is a common fruit (your starting fruit), or native fruit (includes coconuts). Common fruit can get you 100 bells each, for a total of 1,500 bells if you fill your pocket with fruit and sell them all. Native fruit are 500 bells each, for a total of 7,500 bells, but if you want to keep these fruit instead of waiting for another chance to get one, plant them in an open space so they can grow in threes. You can easily get native fruit by mailing neighbors common fruit, and they may give you native fruit, or visit another player's town and collect their fruit if they are different from yours. Coconuts are also native fruit, but they are rare. You can get them from the shores of beaches in your town. Coconut trees only grow on shores, instead of where any tree would grow.
Afraid of losing your only native fruit to the elements? Here's an easy way to guarantee growing a native fruit on your first try:
There are only a few certain spots in your town where trees will grow. If you don't want to lose your rare native fruit to a possible 'dead' patch of land, simply cut down one of your trees that is unimportant to you with an axe (such as a cedar tree) and plant the native fruit in its place. After the tree matures, you will receive a native fruit-bearing tree with three native fruits on it, so even if your next attempt at growing more native trees doesn't go well, you'll still have a steady supply of the fruit.