I don’t go into high contrast mode very often, but need to when testing a web application for accessibility. I always have to look up the key combination that toggles it on and off, though! The keys are shift, alt and printscreen, all pressed at the same time.
One note about high contrast mode. Background colors you have defined with css go away. So if you have popup menus or other popups on your page, you will see the page underneath through the popup. Messy. The only way I know to get around this is to position an iframe behind the popup, setting it to the same width and height.
UPDATE (10/4/2010): Be sure to refresh your browser once you switch modes. Also, some programs will require a restart to get them fully out of high contrast mode (notably, Eclipse-based programs) when you switch back to normal mode.
If your system seems to get stuck in high contrast mode (once I go into HC mode on vista, it seems to reboot to high contrast mode even though I shut it down in normal mode), right click on your desktop, choose properties, then themes, and change your theme to another theme and then back again. That seems to do the trick of clearing out the lingering high contrast trigger.