Close Blinds and Curtains
Inexpensive mini blinds or curtains can work wonders for reducing the sunlight and heat streaming in through your windows! At my house, installing inexpensive blinds on the south-facing windows completely changed the temperature in that part of the house.
Make sure blinds or curtains are white on the side facing the outside. Solar sun screens and window film are other options that can greatly reduce the heat coming through windows.
Install Cool Lighting
Incandescent light bulbs can significantly heat up a room. Replace standard bulbs with high-efficiency, low-heat CFL (compact fluorescent lights) or LED (light emitting diodes) bulbs, and turn off lights when not needed.
Turn Off Electronics
Computers, TVs, and other electronics generate quite a bit of heat when sitting idle or even when turned off, so unplug devices when not in use. An easy way to do this is to plug electronics into a surge protector which has an on/off switch, then turn the switch off when the devices are not in use.