How Can I Save Bandwidth?

I need help about how I can reduce bandwidth usage in an internet cafe.

I am an I.T Officer for a church that is running a not for profit internet cafe in Africa and I have a big problem with bandwidth usage. My I.S.P provides about 3GB for 530 US$ per month. In the last few months, we have been using about 5.9GB and we are being restricted now which makes it impossible for us to work. The I.S.P. wants us to pay 900US$ which is basically impossible for us.

What I need is help on how I can reduce bandwidth usage in the cafe without compromising our quality of service. Please any ideas, help will be highly appreciated.

Thanks