how to configure two isp in one Router

Dear all,
I have a cisco 2511 router with 2 serial and 1 ethernet interface.we are planning to implement two isps on the same router.
Can you tell me how i can do this ,is it possible in this Router.
My intention is to have both the lines in the same Router,so that even when one line is down,my users can still work on the internet….
what is the best possible solution for that.