Magia, here's the recipe I have always used for popcorn balls. In my opinion, the "trick" for getting the popcorn to remain fluffy and not get soggy or stale is to be certain to completely coat the popped corn with the syrup, and you have to work fast to get this done. One the syrup is poured over the popped corn (by the way, do this in the largest bowl you can find!) and stirred with a wooden spoon, it cools very quickly, so I get in there with my hands, well buttered, to mix some more.
Popcorn Balls
5 quarts plain popped popcorn (about 3 bags of microwave stuff)
1/4 cup butter
1 cup packed light brown sugar
1/2 cup light corn syrup
2/3 cup sweetened condensed milk
1/2 teaspoon vanilla extract
In a medium saucepan with a candy thermometer inserted, combine butter, sugar and corn syrup. Stir well and bring to boiling over medium heat. Stir in condensed milk; simmer, stirring constantly, until thermometer reads 238 degrees F (114 degrees C). Stir in vanilla extract.
Pour caramel over popped corn and stir to coat. Butter hands lightly; shape popcorn into balls about 3 1/2 inches in diameter.