The first girl that ever did anything for me wouldn't let anything down her throat and didn't want me to touch her head.
So when the second girl did it, I didn't touch her and warned her a couple times before I was about to come. Finally she picked her head up and said "I'm gonna swallow it, shut up." Afterwards she asked me why I didn't touch her head or show any appreciation.
Women are different, find out what she considers proper etiquette. :-)
I think the most important thing is to appreciate it and let her know how much you do--by not demanding anything, and returning the favor without prompting.
