You know what they say, different strokes for different folks (except for the guy on the top-right). And the same thing can be said for women and their breasts. No two racks are alike, but here are a few techniques to test out based on type and size:
Large Breasts: Researchers from the University of Vienna found that large breasts were about 24 percent less sensitive than small ones, which is most likely due to the nerve that transmits sensation from the nipple is stretched. Stimulate the outer sides of her breasts, just below the armpits, with your tongue or fingertips. Make flipping motions with your tongue and even experiment with light nibbling.

Small Breasts:
They're sensitive, but they can handle more motion because of their size. Use your palms to cup and gently bounce her breasts during sex.

Droopy Breasts:
Droopy breasts are likely less sensitive, mainly because the nerves are stretched or compressed by the breasts' weight. Have her lie on her back to help to shift her breasts up and out. This helps in relieving the tension on the nerves and letting her focus more on the pleasure.

Surgically Enhanced Breasts:
Implants don’t necessarily interfere with sensation, but they do tend to move differently. Concentrate more on the surface of her breasts. Use your tongue to make circles that gradually spiral in toward the nipples.

New-Mom Breasts:
Focus on the breasts' undersides instead of her tender nipples. Gently cup and support her breasts. It'll feel nice to her after a long day of suckling.