Now to the modern day: some of the nicknames that were given to New York City and are still used today are "The City that Never Sleeps"--this is because New York City always has something going on 24 hours a day 7 days a week. Frank Sinatra had a song that became a slogan and finally a nickname: "If you can make it there you can make it anywhere." There are many opportunites for people to make it in New York City so this caught on like crazy.
But for complicated reasons in the 1970's New York City went through many fiscal troubles. The city became the butt of many jokes, and there was a big drop in tourists coming to New York City: many people, in fact, left New York City. This led to a campaign to give New York City a new image. So Charles Gillate, who worked for the New York City Visitors Bureau, used the nickname Big Apple to draw tourists to New York City.
This idea came from the 20's and 30's when musicians had an expression for playing in small town gigs. It was that 'there are many apples on the tree but when you pick New York City you picked the Big Apple.'
This nickname, the Big Apple, helped turn New York City around. People wanted to see what was with this Big Apple and returned to New York City. Based on reports for 1999 New York City is number 2 for tourists: only Orlando is ranked above New York City. So the Big Apple nickname has paid off. With Disney moving to 42nd Street New York City may be number one in tourism soon.