How do you get air conditioning to flow upstairs only?

I currently have one A/C unit outside and it runs for the entire two story house. Downstairs is already cool but upstairs is hot and I would like the A/C unit to focus on getting upstairs cool only to conserve power because we are rarely downstairs and it doesn't get as hot as it does upstairs. Can this be done? What will I need to do to get this done and at what cost? Is there a installation term that I can ask for when shopping for an A/C repair guy. Thanks.