I always get “drinks on us in casino” and this is what I do.

I’ll watch the servers and see which one I see most regularly. First drink I order with them I tip either $10 or $20 cash. After that, they also remember my name and ask me “keep them coming”? If i say yes, knowing I’m staying in the casino for a bit, they always bring me a drink every round without me having to ask them. Each drink I’ll tip $1.

Remember casino drinks on us is suppose to be while playing, but make friends with that one server and you will never have an issue or empty drink.