If you keep tripping and falling in Animal Crossing: New Leaf, it means that you're having bad luck today. You can improve your luck in a variety of ways. This page explains the details.
Tripping is caused by bad physical luck (see below). To stop tripping, you need to improve your luck. One guaranteed way to improve your luck is to get your fortune told by Katrina and wear the good luck item that she recommends. If Katrina is not in your town right now, you can visit her in another town. Also, you can get Katrina to open a fortune teller shop in Main Street after she has told 20 fortunes while in your town. See the shop unlock guide for more details.
Another way to increase your luck is to put green items in the southern parts of your rooms. Read the Feng Shui guide for more details. Note that Feng Shui might not actually stop you from tripping and falling, but it might help.
In Animal Crossing: New Leaf, your luck is random every day. Different good or bad things can happen to you based on your current luck. There are 5 types of luck: money luck, friendship luck, love luck, item luck, physical luck. Each of these can be good or bad each day. This has the following effects:
|Luck Type||Good Luck||Bad Luck|
|Money Luck||Trees drop 200 bells instead of 100||Trees drop fewer than 100 bells|
|The money rock drops 200, 300, 500, 1000, 2000, 4000, 8000, and 16000 bells||The money rock drops 100, 100, 200, 300, 500, 1000, 2000, and 4000 bells|
|Villagers give you more money||Villagers give you less money|
|Friendship Luck||It's easier to increase friendship with same-sex villagers||It's harder to increase friendship with same-sex villagers|
|Special conversations with same-sex villagers are more likely||Special conversations with same-sex villagers are less likely|
|Love Luck||It's easier to increase friendship with opposite-sex villagers||It's harder to increase friendship with opposite-sex villagers|
|Special conversations with opposite-sex villagers are more likely||Special conversations with opposite-sex villagers are less likely|
|Item Luck||More likely to get Saharah-exclusive items||Less likely to get Saharah-exclusive items|
|More likely to get new items from item giveaways (Easter, Fireworks Festival, Snowman, Fortune Cookies, etc.)||Less likely to get new items from item giveaways (Easter, Fireworks Festival, Snowman, Fortune Cookies, etc.)|
|More new seashells appear on the beach||Fewer new seashells appear on the beach|
|Villagers more likely to offer something for sale||Villagers less likely to offer something for sale|
|More likely to get rare items in Leif's weeding mini-game||Sitting in a chair will make a sound|
|Physical luck||Encounter fewer jellyfish||Encounter more jellyfish|
|Bees move more slowly so are easier to avoid||Bees move more quickly so are harder to avoid|
|Fewer mosquito bites||You will often trip and fall while running|