Instituting rush charge for ad agency

Greetings:

Can anyone here give some advice on instituting rush charges for a
business? I work for a small advertising/public relations agency and
we have several clients who often call us in a panic needing something
done right away or within the next day or so. As a result we end up
working insane hours to accommodate the requests into our already full
schedule. One of our clients who does this the most used to work for
an agency and actually told us she can't believe we don't charge her a
rush charge.

Is there any standard for rush charges, such as a percentage of the
cost of the service, or a set fee?

Any advice would be greatly appreciated. 

Cheers,
Jeremy