Lily and Gorden are both right. But the answer is not unique. You can call middle aged women " 小姐”too. But some people, whether old or young women dislike to be called as"小姐”, because 小姐 sometimes means prostitutes. ( She is a 小姐。= She is a prostitute. so calling " 女士”is safer for you . haha . Calling girls " 美女” is very popular in Guangdong. Even if the girl is not pretty, you should call her " 美女=beauty. 靓女/美女=beautiful girl This is how people call girls in Guangdong. And it is better to call waitress 美女/靓女 too. This makes the girls feel good about you that you don't remind them of the fact that they are waitresses. Maybe in other places , there are different words, but 女士 is the FORMAL STANDARD CHINESE word. It's OK in verywhere I think. About 姑娘， I only heard Chinese people from the North part call girls like this. I used to be call as a " 小姑娘”with a strong north part accent by a north part man. haha This is a good word, nothing bad. Usually south part people don't use it , it is just a habbit.

in restaurant you can call a girl 美女、服务员 that are better than 小姐，as there is another meaning for 小姐,which means prostitute. on the street, you can say"（前面）那位女士，你钱包掉了" then she will turn back and appreciate you very much hehe^_^ 姑娘is used in some parts,such as northeastern areas.if you are in the restautant of China's northeast region,you can call 姑娘 loudly with your hands waving to her, just like northeastern people who have liberality and generosity in their characters. Besides all above, in Sichuang local people call a girl幺妹儿（yao mei er）in restaurant ,spoken out by their non-national accent hehe^_^

小姐is used for young woman，女士isfor middle-aged woman ！"小姐, you drop your wallet"? can translate like this"那位小姐/女士，你的钱包掉了！"In restaurant,we can call the young woman server 服务员In general .

In restaurant，you call 服务员。 if i saw a woman 's wallet falling on the srteet,you would say :你好 ，you drop your wallet。小姐一般都是很书面的语言

正常场合下是还好的