Breasts and sex (advice for men)
Part of being an adult is accepting responsibility for your own sexual pleasure. Our bodies are all different, our pleasure triggers are different, and our preferences are different. Some women love to be touched on their breasts and others don't. So obviously, any advice offered here is from my personal view - other girls may have entirely different ideas, and I cannot claim responsibility if your woman doesn't like the same things as I do.
Don't talk to her breasts, but talk to her about them. Tell her that the size is just right - chances are you won't be lying, considering you obviously fancy her. Anyway, the qualities of her breasts are like religion. If you really love her and it's important, you'll convert. Feeling that her body turns you on will be very satisfying for her. Comment on how soft the skin of her breasts is, and how kissable they are. It helps the situation if she can feel relaxed and comfortable about her body.
When it comes to activity, a good rule to remember is not to believe pornography. Porn is geared towards turning guys on, not giving them any real idea what a woman likes. Trust me.
During foreplay, pay attention to a girl's breasts, and continue to do so as you get more intimate - it doesn't do to give each breast a cursory check and move straight in for action lower down. Women who say their breasts are sexually responsive like a firm -- even rough -- touch, such as kneading or pinching, as arousal mounts. But it can't start out that way. It's too painful. Women prefer to begin with a caress (you can squeeze harder as she gets more turned on or requests it). I love to have my breasts caressed and cradled in my lover's hands, and to have him kiss them (though the nipples need to be left alone for the time being, until I'm turned on) - it makes the encounter far more enjoyable to look back on, too.
Mouth-to-breast contact can be great, but again, you have to stay gentle (less so when she's very turned on, but you still don't want to be too rough - it's desensitising). Round and round the nipple with the tongue tip, and gently sucking the nipples, combined with soft kneading with both hands will usually be a winner. Just remember to keep returning to the breast - it'll make it more interesting for her.
If the idea turns you on, encourage her to try breast sex with you. The experience is very intimate and can be very rewarding for her as well as you (for more detail consult the 'sex with breasts' article). Contact between the head of your penis and her nipples can be quite stimulating, even apart from the physical feeling.
Really satisfying sex is not easy. It's a learned response. It takes patience, practice, willingness to learn, and above all, communication. So explore the alternatives, and enjoy the opportunity for the two of you to communicate about what you both enjoy, and how you both like to be touched! It's absurd to think that you could figure all of this out about her by intuition. In whatever gentle way you can find, tell her that it's her job to tell you what her needs are, just as you'd hope she'd want to hear from you about what you desire. Finding out just where and how your partner likes to be touched and letting her know what you like is a delightful part of getting to know each other. If she doesn't know what her needs are, encourage her to explore on her own or with you.