Drinks are on the house!
Obtaining complimentary beverages and alcoholic drinks in casinos is all about gambling. Either you give the impression that you are gambling or you actually play the slots. Once you have taken a seat at a slot machine, the cocktail waitress only needs the impression that you are gambling (and 21 or older) before she takes your free drink order. Cocktail waitresses are constantly walking through the maze of slot machines in all major casinos, looking to take drink orders. Place your order and she will return with the drink of your choice. If you give a tip (usually $1 or $2 per per drink) they'll come back again and again to take your order. Free drinks are also provided at table games.
Although this is technically free, you really are expected to be actively gambling and leave tips. The decision to play or skirt around the rules lies entirely in your hands.