you can't if you are on a broadband or dsl line. dial up used to give you a different ip everytime you signed on, but usually ips are static now.
A different internet source--like a cell phone hotspot or wireless wifi from a neighbor would give a different ip, or you can use your same internet and go through a proxy server. just google proxy server and the second and third results most likely will be sites that will let you hide your ip.
nate might know more about it, as far as how they screen ips though