A head rub is a delightful experience, for both the giver and the receiver. It's not completely self explanatory - here's my process:
from any point on the receiver's hairline, comb your fingers through the hair and stay close to the scalp. Then gently tug your fingers through the length of the hair. Repeat. This will generally put people to sleep (or at least lull them into total complacency) if done for more than 30 minutes.

If you're the nurturing type, it's a turn on to have someone drooling in your lap like a baby. If you enjoy being nurtured, hey... find someone with clean hands and a soft touch and read them this node.