The amount of stamina you have is located as a green bar in the lower right-corner of the screen. As you work with your tools, the bar will decrease until it turns red and reaches zero. If you deplete your stamina bar then your character will pass out. At the beginning of the game you will have a max of 600 stamina points.
Throughout the game you'll find Power Berries that will increase your overall stamina. There are a total of 8 berries in the game. As you obtain each one you'll eat it and immediately be able to use your higher stamina level. Each Power Berry is worth 50 stamina points.
When you collect all of the possible berries, you'll have a total of 1000 stamina points.
Power Berry #1: At the start of the game
As you start through the game and meet the Harvest Goddess, she will give you a starting berry.
Power Berry #2: Max friendship with an animal
On your farm you'll have barns set aside for poultry and livestock. You can buy the animals from Horn Ranch. As you take care of your animals, they will increase the number of hearts each one has for you. When you get 10 hearts with an animal you will receive the berry.
Power Berry #3: Garmon Lower Mine 30th floor
At the very bottom of the Garmon Lower Mine you'll find two islands; a large one that contains the staircase and a smaller one in the northeast corner. The smaller island is part of the Treasure Map sub-quest. The Power Berry is hidden in the lower left corner of the larger island. You can find it by walking around the area and pressing the A button over and over.
Power Berry #4: Garmon Upper Mine 45th floor
After you ring the Red Bell and light the path to the upper mine, climb up to the Harvest King's seat on floor 45. When you go up to the King's platform, press the A button on the northern side of the icy cliff just before you reach the platform.
Power Berry #5: Gain max level with your Fishing Pole
Toby will be the one who gives you a fishing pole. You can meet with him at the edge of the river in Flute Fields, next to the waterwheel, and get a free Fishing Pole.
As you use the pole its level will increase. You need to reach skill level 5 before you can receive the berry. Leveling up the pole will take some time, and you can not stand around spamming the A button and expect it to increase.
The amount of experience you earn when using the Fishing Pole will depend on the difficulty of the fish you catch. The stronger the fish, the more experience you'll earn. The game does not tell you the strength of the fish but if it takes many A-button presses when reeling it in, chances are it'll be worth a lot.
The legendary King Fish are the strongest fish (and worth the most exp) that you'll find. The Barracuda, Dorado, Giant Arowana, Tarpon, and the Nautilus fish are worth 130 points each. The easiest items to catch are the Empty Can and Algae, which are only worth 2 points each.
You need at least 99,000 experience points to max out the Fishing Pole's level
Power Berry #6: Gain max level with your Hoe
The Hoe is much easier to level up than the Fishing Pole. You even start your farm life with a Hoe and Watering Can from Hamilton.
You will need to reach skill level 5 with your Hoe in order to earn the Power Berry. This tool can be leveled up by simply equipping the Hoe and repeatedly pressing the A button. You also earn points by tilling soil that has not already been tilled.
You need 43,200 points to reach skill level 5 with the Hoe.
Power Berry #7: Gain max affection with your spouse
Each of the marriage candidates a max number of 10 hearts, and you can propose marriage after you have reached 9 hearts.
After your wedding ceremony, your spouse will increase the number of hearts he/she has by another 10, making the maximum number of hearts set at 20. You don't get a second group of 10 hearts, but instead your spouse's heart-markers on the Villagers page will be different colors. The lighter colored hearts represent 1 heart, and the dark red hearts are worth 2 hearts. If your spouse had 6 light-hearts and 4 dark hearts, then he/she has 14 hearts total.
You can increase the number of hearts by talking to your spouse daily and giving gifts. Any of the date-based festivals, such as the Flower Festival and Moon Viewing, will go towards increasing the affection with your spouse.
As soon as you receive your last heart with your spouse you'll be rewarded with the Power Berry.
Power Berry #8: Gain max affection with your child.
This berry can be earned the same way that you earn the berry from your spouse, but your child only has a maximum affection of 10 hearts. Either one of your children can be the trigger that gets you this berry. You just have to give gifts and talk to him/her daily and eventually you'll earn the Power Berry.