There are several ways to keep your house cool and save money during hot weather. These can be used to supplement your air conditioning so it doesn't have to work as hard.
Image Flickr Per Ola Wiberg
Vent the top
Warm air naturally rises in a house and colder air settles. That's why your upper floors will normally be hotter than the ground floor during the Summer. If you can vent the warm air out of the top, it creates a natural convection current in the house that pulls the cool air up. When I was young, we didn't have air conditioning. My parents installed an exhaust fan upstairs that pulled the hot air out. In our current house, we have vents in the top of the attic and installed an automatic attic exhaust fan that comes on at 110 degrees (43C) to pull the hot air out. This venting saves money on air conditioning by keeping the air conditioner from coming on as often.
Close the blinds
Keep the sun's heat out of the house during the hot Summer months by keeping the blinds or curtains closed during the day. This keeps the inside cooler and keeps the air conditioning from running. Seal any cracks by caulking around window frames to keep hot air from leaking into the house. You can get reflective window films that reflect the sun's rays away, but in the Winter you want the solar heat to come in and warm the house naturally when it is cold outside.
Ceiling fans help keep the air in the house circulating, which makes the house feel cooler. There are Summer and Winter settings on the fan. The Summer setting pulls the cool air up, and the Winter setting pushes the warm air near the ceiling down. If you don't have air conditioning, ceiling fans can make the house feel much cooler because they keep the air moving.
You can add shade to the house to help keep it cooler in Summer. In the northern hemisphere, plant deciduous or broad-leaved trees on the south side of the house to shade it during the Summer. In the Winter when the trees shed their leaves, the sun will be able to get through and warm the house when you need the natural solar heat. If you live in a really hot climate, you can add exterior shades over the top of the windows to help block the hot sun coming straight down during the Summer. In the Winter, the sun comes in at a lower angle so the shades don't block that much.
If you have a cool basement, you may feel cool air coming up beneath the basement door. Take advantage of this situation by leaving the basement door partially open during the Summer. In Winter, you may want to install a draft guard under the basement door to block this cool air coming in.
I hope these tips help you keep your house cool and save a little money.