over lease miles charge?

Just had my new 2012 7 series bmw for 4 months now and it already looks like I'm going to be driving more than I had planned during the lease period. What is the best way to approach getting more miles added to a lease? Do I just wait till the lease is over and pay whatever? Seems like I heard once that you can have miles added to a lease during for less than at the end but I'm not sure. How do I even find out how much it will be per mile over?