The general consenus is to keep your shades completely drawn while you're away, then to use a box fan facing out in one area of the house and then either leave other windows open or—if you've got 'em—put more box fans in other windows facing in. Either method should create a cooling draft of moving air.
One reader suggested that you only use the fan method at night when you're moving cool evening air inside, then in the morning close all the windows, draw all the shades and turn on indoor fans to trap the cold air indoors.
Also another tip: Balanced humidity levels will help to keep you cool too. If you live in a super-dry climate, hang wet towels around the house. If you live in a wet-climate, get a de-humidifier.
Or there's always the option to make your own portable air conditioner out of an old cooler!