There are few things that site uses to attract members which are (1) uniqueness (2) fairness and trustworthiness (3) giveaways and I think apart from these with time sites get more and more trust and hence members because last thing members don't want is being scammed.
1. I don't think you can find a "unique" website online. Simply for the reason that people just copy+paste other people's idea. One day, you're the only one that have it. The next day, every single one has it.
2. I think gambling sites that have been running for a long time already have proven their trustworthiness. (I can't think of casino slots website, so...) Example of this is PrimeDice, Crypto-Games, etc. The fairness can be proven when they share their codes for how the games are generated. Most sites show theirs.
3. Giveaways are mostly for starting gambling companies. But for older gambling companies, they do promos or challenges like, hit the jackpot and you'll get this x amount of money, or wage x amount of BTC and be a part of this raffle, etc.
