Faron Woods - Piece of Heart Location
There are two pieces of heart for you to find in the Faron Woods. The first of which you find extremely early on in the game, it's literally the first piece of heart you're able to acquire. The second piece of heart you find in Faron Woods is much much later in the game.
Giving more information may be considered spoilers at the point in the game most people will be reading this page. Since that's the case - you'll have to scroll down past the first piece of heart for more information on the second!
Faron Woods - First Heart Piece
In the Faron Woods there is a cave to the north east (apparently north west on the Wii) that you visit early on in your adventure for a Small Key. Along side the chest that has the Small Key in it are two torches, you can see them in my screen shot. Light both of them to make a secret chest appear on the ledge behind the first treasure chest.
If you missed this piece of heart prior to getting the Wolf form you'll need the Lantern to clear the fog in Faron Woods to reach this location again.
South Faron Woods - Piece of Heart
You can't reach this piece of heart until much later in the game when you get the Dominion Rod and return to Faron Woods for the Owl Statue. Follow the path north from the south Faron Woods Warp Portal until the path splits just after Coro's shop.
One of the two paths has a boulder blocking it - or used to if you destroyed it earlier in the game. Take this path and you'll find yourself in a small alcove that has an indentation in the ground and an Owl Statue in the back to move, as seen in my screen shots.
Move the Owl Statue into the indentation and then turn into a Wolf. Climb onto the nearby rock and Midna will guide you across a couple of jumps to where the piece of heart is found.
