It's pretty straightforward: If the temperature in your house is lower than the temperature outside, leave the windows closed; if it's warmer than the air outside, open the windows. You're after the coolest air you can get; you wont' get cooler by opening the windows when the air outside is hotter.
If you have the windows closed, it's a good idea to have a fan blowing, preferably onto your torso; that will help move the air around, and moving air helps "collect" body heat and move it away from you. Think about a cold but windless day - it doesn't FEEL nearly as cold as a cold and WINDY day.
Also, be sure to keep yourself hydrated; sweat occurs for a reason, and you want to have plenty of fluid available to produce the sweat.
So keep the air moving, drink plenty of liquids, and pray for the a/c guy to show up FAST :-)