I think a neg that I have found to work well with everyone I have field tested it on is the laugh neg.  Everyone has their own unique laugh so I love using this one.  A personal favorite is comparing a girls laugh to that of Yoda, yes the character from Star Wars.  Theres plenty well known to compare a girls to, a few I use are Yoda as previously stated, Tigger from Winnie the Pooh (girls love this one), the unmistakable Fran Drescher, Woody Wood Pecker, and Ricky Ricardo from I Love Lucy.  Normally I will point out the similarity to the target and the rest of the set, and if I'm feeling it I will do a brief impression, mocking the target.  Most of the girls I have used it on often tell me that they think of my impersonation when they laugh for weeks following.

That's a good idea and it's not overtly mean. I used to have a female roomate and she had one of those infectious laughs that she would change from squeeky to guffaw, I would impersonate her laughing and the more she heard me make fun of her laugh, the more she laughed. It's a great way neg and not lose a person. And everyone loves to laugh, especially at themselves. It makes you feel good. Not only that but it usually attracts the attention of others so if that person stops laughing and negs you often there is another person standing near you can turn to and to, that often will tickle their jealous bone.