Sacrificing two Damsels (living or dead) is the fastest and safest way of earning 16 favor. Due to the great increase in survivability granted by the Kapala, it is usually worth missing out on a kiss or two in order to gain enough favor quickly.
It's a good policy to sacrifice any Damsel that appears on a level with an Altar if you don't yet have the Kapala, unless you're attempting to unlock the Changing Room, or have extremely low HP and there aren't enough substitute enemies in the vicinity to make up for it.
Once equipped, it allows the Spelunker to absorb drops of blood by touching them - much like the Vampire does - and gain one hit point for every eight drops collected. The icon at the top of the screen illustrates how much blood is needed before the Spelunker obtains more health.
An easy way to harvest blood for HP is to find any form of enemy which leaves a corpse, such as a Caveman, Hawk Man or Yeti, and stone them to death with a rock.
Even after the enemy is dead, the corpse will still bleed when hit with the rock; humanoid corpses are able to yield up to 8 blood after death. The more corpses you can stack in one spot to throw the rock, the more blood will be produced at once.
Just be careful of ricocheting the rock off a wall and hitting yourself, and don't get too carried away or risk facing the Ghost.
Note that a corpse placed on an altar vanishes in an explosion of blood. If you have the Kapala, standing on an altar while you make a sacrifice enables you to collect most of the blood in one go.
The Hindu goddess Kali, among other Hindu and Buddhist deities, is often depicted holding or drinking from a blood-filled Kapala.