At the beginning your farmer doesn't have much stamina. You'll be able to work for a bit, but eventually he'll get tired and you'll have to stop working. To raise your strength you'll need to find Power Berries. There are 10 berries that you can find in various locations.
PBerry 1 » In your field
Simple enough! Take your hoe and dig around your crop field. The berry will eventually turn up.
PBerry 2 » Harvest Goddess
Take items you've grown on your farm to the Harvest Goddess. Toss the item into the water to make her appear. After throwing in an item for 10 days she'll reward you with a berry.
PBerry 3 » Turn in medals
When you bet on the horses during the derby race, you can win medals. Exchange 900 medals to Mayor Thomas to get the berry.
PBerry 4 » Win the Horse Race
Your horse will become an adult at the beginning of your second year. You can now participate in the Spring and Fall horse race! Yeehaaw! If the two of you come in first place you'll win the berry.
PBerry 5 » Frisbee disc winner
In your second year you can have your dog join in the Dog Frisbee event on Summer 01. The prize is a Power Berry, so practice throwing the frisbee with your dog when it becomes an adult in the Fall of your first year. You will also have to buy the Frisbee from Won for 5,000 G.
PBerry 6 » Fishing off the pier
When you upgrade your fishing pole to level 5 (mithril), use it at Mineral Beach in the Winter. You might just catch a berry.
PBerry 7 » Dig down into the mine
Next to the Harvest Goddess' pond is the mineral mine. Usually you go here to get ore for tool upgrades, but if you did down far enough you'll find a berry. Actually the berry is down on the 100th floor and you'll have to dig it up with your hoe once you get down that far.
PBerry 8 » Television Shopping
Once you've purchased most of the items off of the Saturday t.v. shopping show, they might just have a berry for sale. It's not cheap! You'll have to fork over 10,000 G.
PBerry 9 » Behind the Winter Mine
When it's Winter time the lake on Mother's Hill will freeze over. Walk across it and you'll see a small island in the middle of the lake. The island has the entrance to the Winter Mine on it, but if you walk behind the island and press the A button you'll find a hidden berry.
PBerry 10 » Dig in the Winter Mine
Pretty much the same as the berry that's hidden in the mineral mine, but you don't have to work as hard for it. The berry is only on the 19th floor this time.
You can earn this berry from Kappa. He lives in the lake on Mother's Hill and he has a fondnes for Cucumbers. If you throw a cucumber into the lake he'll pop out for a second, then return back into the water. Throw in a Cucumber again the next day and he'll emerge and call you persistant. Keep throwing in a Cucumber for a total of 10 days. On the 10th day he'll give you the Special Berry in hopes that you'll leave him alone.