According to me, the current problem is not with creating more ideas for reusable cups but creating ideas to get people to USE the cups. This is very simple, yet effective when implemented problem.
Touch people where they are most sensitive ie MONEY. If the regular price of coffee is $4 then instead of offering a $.50 discount on the coffee when you bring your own cup, offer a $1 surchage ie make the coffee $5 if they DO not bring a coffee cup. This had been proven by research, that people avoid loss rather than look for gain. This can help use the buying behaviors of customers for the better of the environment.