Which of the following does NOT help increase warmth in a sleeping bag?

Sleeping on clean bare soil does not contribute to increasing warmth in a sleeping bag. In fact, it can have the opposite effect. The ground tends to absorb heat from your body, and when you sleep directly on it, especially if it's cold, it can draw warmth away from you. This can lead to a significant loss of body heat, particularly when sleeping outdoors in cooler conditions.

In contrast, using a sleeping pad serves as insulation between your body and the cold ground, which helps retain heat. Wearing layers of clothing is also beneficial as it traps warm air close to your body, and keeping the sleeping bag dry is crucial since moisture can lead to heat loss, dramatically reducing the bag's insulating properties. Thus, while the other options effectively contribute to warmth, sleeping directly on bare soil does not provide any advantages in maintaining body heat.