Pretty easy. Go to www.whatismyip.com and write down your IP address. In Analytics, look at the menu at the bottom of the screen and click "Filter Manager". Add a new filter, give it a name, select "Exclude all traffic from an IP address", and choose which website profiles should have the filter. Easy!
Depending on your service provider, it is possible that you get a new IP address every time you shut down or reboot your machine. If that's the case, you'll have to go in and edit your filter each time.