In the United States, it's a federal law that all sweepstakes provide the disclaimer that a purchase is not required to enter, nor will it increase their chances of winning. Over time the phrase "No purchase necessary" (along with its sibling, "Void where prohibited") has emerged as the most succinct way to make this disclaimer.
Usually it's put in tiny print somewhere near the bottom of the junk mail advertisement, or read off as fast as possible by an announcer at the end of the radio commercial. A new law passed on the last day of the 1999 Congressional session requires sweepstakes promoters to place this disclaimer in a more visible location, following a rash of consumer complaints and lawsuits against supposedly deceptive promoters. (One such lawsuit against the well-known Publishers Clearing House has been so costly, they've been forced to file for bankruptcy protection.)