After defeating the boss of the Yeti dungeon, Yeta and Yeto will hug. Almost 300 hearts will appear. Keep talking to them for more harts to come out.
When low on hearts, go back to a spring whose spirit you’ve already returned (like Ordona’s spring in Ordon for example). Walk around inside the spring and your hearts will slowly recover. You can also go to the Hot Springs at Death Mountain to regain lost heats.
Go to Castle Town after you save it from the Twilight. Head to the west side of town and find a bone. Now go south with the bone to find a dog. Throw the bone into the alley way and the dog will come out with either hearts or rupees.
Talk to the three girls outside the tent in Castle Town after beating the Star mini-game. They will giggle over Link and drop hearts.