Wagering contests are something that I think would be worth trying, as they are self financed while players are competing for it. Giving away something like 0.1% of the wagered monthly amount would create quite big prizes, while wagered BTC would probably be quite bigger than with no promotion. For example, from January 15 to February 15, 21293 BTC were wagered, so there would be 21.293 BTC in prizes. I doubt wagered wouldn't increase by more than 21.293/0.0075=2839.07 during that time. Also consider that there would probably be a positive effect after the promotion that wouldn't occur otherwise. Some new players would join who would continue playing, and contest prizes are likely to be gambled again, so they are unlikely to end up being a complete loss for the casino.
Also wagering contests encourage low multiplier bets, which are preferrable for the casino as they cause less volatility than high multiplier ones.
Unfortunately the math is not quite as favorable. bustabit's EV is 0.25% of the wager volume, not 0.75%. So for a wager contest to pay for itself, the wager volume would have to increase by about 66% rather than ~13% during that time frame. That's not to say that a wager contest couldn't be a success, of course.
The fact your customers found their way to Bustabit via word of mouth speaks volumes.
Which chat software are you using?
I'm not sure if that's what you mean, but bustabit doesn't use any external chat software.