Sorry dude, but I'm afraid you're going to have to learn through experience. (But what experience!!!)
Not only are women's individual preferences widely different, but I'm guessing many women are like me - what feels great one day (yeah baby, more of that!) might just be hohum some other time. Sometimes I like having my nipples pinched really hard; other times they're more sensitive and pinching hurts and is a real mood-breaker, but light brushing is more er...effective. I ALWAYS like having the undersides of my breast played with. IMHO, and according to most women I know, that area is a much-neglected erogenous zone. But other than that, I think you're going to have to find out what your lady friend likes by asking her or experimenting. Get to work!
