How to change the background of your inventory.
1) Grab a shirt or pattern.
2) Drag it to the lower left corner.
3) It'll be over an invisible space, drop it.
Hold 10 extra items
This is a really easy trick. Just buy 10 letters, leave them blank, than pull an item onto it so that it becomes a present. Tada, 10 extra slots.
Store more items
Speak to Pelly in the town hall (or Phyllis, if it's at night) and choose "Save a Letter". You can save up to 75 letters, and you can attach gifts/items to these letters so you can keep 75 items in the post office, plus the 90 items in the dressers, and you can do this with all 4 characters, for maximum item storage.
Getting furniture from your neighbors houses
If an animal in your town has furniture in his or her house that you want, send them seashells, unwanted furniture, or fossils to them in the mail. They'll usually put these gifts in their house, displacing whatever was there before. Check the recycle bin at your Town Hall and the item you wanted from their house may be in there.
Instantly Cure Sick Villagers
When one of your villagers is sick, you'll have to give him/her some Medicine every day for about a week to cure him/her, but an easier way to cure him/her is to get on WiFi or multiplayer, then the villager will be immediately cured when someone visits your town.
First Roof Color
Your first house's roof color can be customized, believe it or not. Simply change your DS's menu color from the DS setup screen and your house will have that same roof color. NOTE: this only works with the color you have set when you first move into a house.
Free Fishing Rod from Tortimer
If you go up and talk to Tortimer at the Fishing Tourney and you don't have a fishing rod with you, he should give you his. It's just a normal fishing rod, but hey, it's free.
Make animals stay in their houses
To make animals stay in their houses, simply go to the gate, and ask Copper to open the gates for DS to DS visitors. After the gates are open, your animals will stay in their houses. This makes deliverys VERY easy if you are having trouble finding certain animals, or if you are trying to look at a certain animal's house and they don't stay inside much.
Find out tomorrows weather
If you have a TV, make sure you pay attention when the weather show is on. It'll tell you, in pictures, how the weather will be the next day. It'll either show a picture with the sun for a clear day, or clouds with snowflakes for snow, or rain etc. This is important if you want to catch a Coelacanth.
Free bug net
While the bug contest is going on, talk to tortimer and and he will give you a free bug net. This only works if you don't have a bug net on you already.
Free Pack of Flowers
Talk to Tortimer during the Flower Fest for a free pack of flowers!