DR. steven Eric Udelove, in his book '100 days practice for Good Health, Better sex and long life has given 4 exercises for tongue, one of them is like flipping like a snake. All this exercises can be used to stimulate nipples. You may find this book review in Amazon.com. It is an informative book on Sex.
If too much sensitive avoid the nipples. you may rotate the whole brest with your palm, allowing nipple come from thumb and index finger.Fully rotating clock wise and anti clockwise or half rotation. Gradually nipples may become less sensitive to pain and more sensitive to pleasure.Different people have different sensitive parts or zones.
Depending upon the person, nipples are sensitive or not so sensitive to varying degrees in both men and women. Go with what your partner finds pleasurable. If hers are very sensitive, avoid those areas.