When you are low on health and are not in the middle of something, go to Southeast Hyrule Castle Town. If you have not already done so, complete the STAR mini-game. This is the game inside the large purple circus tent. Afterwards, talk to the three girls directly outside. After they "freak out" about you, three hearts should appear. You can repeat this as many times as desired.

When your health is low and you see a fairy, take out your bottle and run into the fairy. While the fairy is circling around, use your bottle. The fairy will heal you and be captured in your bottle.

When your health is low, catch some fish. You can do this as many times as needed.