A nice, short NAA blog post about why contests and raffles are okay, but not lotteries:
As an apartment marketing professional, it’s important to understand the differences between a contest, a sweepstakes and a lottery—because while the first two are legal, a lottery is not.
When defining the differences, there are three elements to consider: chance, consideration and prize. If two are correctly included, your promotion is legal; if all three are, it’s not.