Boiling the lemons first softens their skins, speeding up the preserving process, and allowing them to release more juice. If you want to spike your preserved lemons with aromatics, tuck a cinnamon stick, a few whole cloves, peppercorns, coriander seeds or a bay leaf (or any combination of these) into the jar before you pour in the lemon juice.
1 dozen thin-skinned lemons (use Meyer lemons when you can get them, usually available at whole foods stores)
1/2 c. coarse kosher salt
Bring a large pot of water to a rolling boil. Add the lemons and cook for 5 minutes, then remove from the pot with a slotted spoon. When cool enough to handle, cut 6 of the lemons lengthwise into quarters and toss them with the salt in a bowl. Cut the other 6 in half crosswise and squeeze the juice out into another bowl.
Pack the salted lemons into a glass jar, and dump in the excess salt. Pour over enough lemon juice to cover them. Seal the jar and store at room temperature for a week, turning the jar upside down each day and giving it a shake now and then. After a week, add a couple of tablespoons of oil to the jar and refrigerate. Preserved lemons will keep in the fridge for up to 4 months. (Or, really, almost forever. They are, after all, preserved.)